Process for preparing spheroidal alumina particles

ABSTRACT

Spheroidal alumina particles have a combination of high macroporosity, surface area, crush strength and low attrition loss and bulk density. The particles are prepared by an improved external gelation oil-drop process. In the process a slurry of alumina, preferably a microcrystalline boehmite-pseudoboehmite intermediate, is prepared in an acidic aqueous medium and droplets of the slurry are passed through air into a column containing an upper body of water-immiscible liquid and ammonia and a lower body of aqueous coagulating agent. Catalysts employing the particles as a support and having catalytic agents distributed at controlled depths in the particle have excellent initial and sustained activity and durability, especially for eliminating pollutants in automotive exhaust streams.

This invention relates to an alumina composition, a catalyst supportcomprising spheroidal alumina particles that may be prepared from thealumina composition, and a catalyst employing the alumina particles as asupport. The invention also relates to processes for preparing aluminaand spheroidal alumina particles. The low density and high surface area,macroporosity, mechanical strength, and stability of the alumina supportprovide a catalyst of excellent activity and durability, especially forconverting atmospheric pollutants in automotive exhaust gases to lessobjectionable materials.

Catalysts often comprise a major portion of macrosize particles formedfrom porous solid support material and a minor portion of one or morecatalytic materials carried by the support. The macrosize particles aregenerally about 1/32 to 1/2 inch in width or diameter and about 1/32 to1 inch or more in length, commonly about 1/16 to 1/2 inch in length.

The activity, efficiency, stability, and durability of a catalyst in areaction depend upon the chemical, physical, and structural propertiesof the catalyst precursors, i.e., the support material and the formedsupport particles, and the nature and distribution of the catalyticmaterial on the formed support. Minor variations in these properties mayproduce substantial differences in the performance of the catalyst.Desirably, the properties of the support material that enhance catalyticactivity are retained by the formed support particles. In general, theformed support and catalyst comprising small amounts of the catalyticmaterial on the support have essentially the same physical andstructural properties with slight differences due to the effects of thethermal activation of the catalyst.

The internal porous structure of the catalyst particles and theirprecursors determines the extent and accessibility of surface areaavailable for contact of the catalytic materials and the reactants.Increased pore size results in greater diffusion rates for reactants andproducts in and out of the catalyst particles and this often results inimproved catalyst activity. However, the extent to which pore size canbe advantageously increased is limited. As the pore size is increased,there is a decrease in the surface area where the reactions take place.A good catalyst should have a balanced combination of high specificsurface area, cumulative pore volume, and macroporosity. Highmacroporosity means a pore size distribution with a relatively highproportion of pores having a diameter greater than 1000 A. Further,alumina and formed alumina with a low density and consequent low thermalinertia will produce a catalyst that will reach reaction temperaturessooner.

Catalyst support material is frequently a porous refractory inorganicoxide, such as silica, alumina, magnesia, zirconia, titania, andcombinations thereof. Alumina is a particularly desirable supportmaterial since it inherently has a high degree of porosity and willmaintain a comparatively high surface area over the temperature rangenormally encountered in many catalytic reactions. However, when usedunder high temperature conditions for long periods of time, overheatingof the alumina may cause sintering and change in the crystalline phaseof the alumina which reduce catalytic activity, for example, due to lossof surface area available for catalysis. Alumina is used as a catalystsupport in the form of a finely divided powder or of macrosize particlesformed from a powder.

Since the physical and chemical properties of alumina are highlydependent on the procedures followed in its preparation, manypreparation processes have been developed in attempts to optimize itsproperties for use as a catalyst support material. Alumina is frequentlyprecipitated by combining a water-soluble, acidic aluminum compoundwhich may be an aluminum salt such as aluminum sulfate, aluminumnitrate, or aluminum chloride, and an alkali metal aluminate such assodium or potassium aluminate. However, the properties of the resultantcompositions after washing and drying have generally been deficient inone or more of the properties of high surface area, macroporosity, phasestability, and low density.

U.S. Pat. No. 2,988,520 to Braithwaite discloses a process for makingalumina of high surface area, good pore volume, and satisfactory densityand attrition characteristics by adding aluminum sulfate to an aqueousalkaline aluminate solution. The precipitation pH is maintained constantbetween 8 and 12 and the reactant concentrations are controlled. Theprocess of U.S. Pat. No. 3,864,461 to Miller et al. produces low bulkdensity alumina consisting essentially of pseudoboehmite by controllingthe reaction temperature, the concentrations of the reactant solutionsof sodium aluminate and aluminum sulfate, the rate of introduction ofthe aluminate into the sulfate solution so that a substantial proportionof the alumina precipitates under acidic conditions, and the length oftime of alkaline aging.

As described in U.S. Pat. No. 3,520,654 to Carr et al., alumina of highsurface area, high porosity, and low density may be prepared by reducingthe pH of a soluble aluminum salt solution to 4.5 to 7, and drying andwashing the alumina product. The patent notes that, although low densityalumina is softer and more subject to attrition than high densityalumina, it shows great advantage as a catalyst support. High density,finely divided alumina with increased attrition resistance for use as acatalyst support may be prepared by precipitation from alkali metalaluminate and aluminum sulfate at a pH of 8.5 to 10, and/or aging at apH between 10 and 11 in accordance with the processes of U.S. Pat. No.3,032,514 and U.S. Pat. No. 3,124,418 to Malley et al.

In addition to retaining the surface area, porosity, and densitycharacteristics of the starting alumina material, a process for theformation of macrosize alumina particles should produce formed aluminawith low shrinkage and high attrition resistance and crush strength.Conventional low density supports are generally deficient in structuralintegrity. Unless stabilized, an alumina particle will undergoconsiderable shrinkage of its geometric volume when exposed to hightemperatures during use. Excessive shrinkage produces unoccupiedchannels in the catalyst bed through which reactants pass withoutcontact with the catalyst.

High attrition resistance provides structural integrity and retention ofactivity under conditions of mechanical stress. During transfer, loadinginto the reaction zone, and prolonged use, the catalyst particles aresubjected to many collisions which result in loss of material from theouter layers. Attrition of the catalytically active layer present in theouter volume of the particles affects catalytic performance and alsoresults in a decrease of the volume of the material in the reactionzone. Volume loss by shrinkage and/or attrition of the highly compacted,tightly held particles in a fixed catalyst bed tends to loosen them andallow for increased motion and collisions during vibration. Once apacked bed becomes loose, attrition tends to increase. During storage,the catalyst is often packed in large tall containers awaiting loading.In order to withstand the forces generated by the weight of theparticles above them, the catalyst must exhibit high crush strength.

The size, size distribution, and shape of the particles affect bothstructural integrity and catalytic activity. These properties determinethe volume of catalyst that can be packed in a fixed bed, the pressuredrop across the bed, and the outer surface area available for contactwith the reactants. Finely divided alumina may be pelletized,tabletized, molded or extruded into macrosize particles of the desiredsize and shape. Typically, the macrosize particles are cylinders ofdiameter about 1/32 to 1/4 inch and a length to diameter ratio of about1:1 to 3:1. Other shapes include spheroidal, polylobal, figure-eight,clover leaf, dumbbell and the like.

Spheroids offer numerous advantages as a catalyst support over particleshaving angular shaped surfaces with salients or irregularities, such asextruded cylinders. Spheroidally shaped particles permit a more uniformpacking of the catalyst bed, thereby reducing variations in the pressuredrop through the bed and in turn reducing channelling which would resultin a portion of the bed being bypassed. Another advantage in usingparticles of this shape is that the spheroids exhibit no sharp edgeswhich will attrit during processing, transfer, or use.

One of the most described methods for producing spheroidal aluminaparticles is the oil-drop method in which drops of an aqueous acidicalumina material gel to spheroids in falling through a water-immiscibleliquid and coagulate under basic pH conditions. A wide variety ofoil-drop techniques have been developed in attempts to providestructural and mechanical properties that would enhance the activity anddurability of alumina-supported catalysts. The density, surface area,porosity and uniformity of the spheroidal product very greatly with thenature of the alumina feed and, along with crush strength and attritionresistance, are dependent on the conditions used in the preparation ofthe feed and the coagulation and gelation steps, as well as subsequentdrying and calcination steps. Internal gelation, i.e. gelation of thealumina by a weak base, such as hexamethylenetetramine, that is added tothe feed before drop formation and that releases ammonia in the heatedimmiscible liquid, is the most common oil-drop method.

U.S. Pat. No. 3,558,508 to Keith et al. describes an oil-drop methodemploying an external gelation technique in which gaseous ammonia isintroduced into the bottom of a column containing the water-immiscibleliquid and coagulates the droplets by contacting their externalsurfaces. The Keith et al. process is based to a considerable extent onthe use of specific alumina feed prepared by acidic hydrolysis of finelydivided aluminum. Spherical alumina particles may also be formed by thehydrocarbon/ammonia process described in Olechowska et al., "Preparationof Spherically Shaped Alumina Oxide," International ChemicalEngineering, Volume 14, No. 1, pages 90-93, January, 1974. In thisprocess, droplets of a slurry of nitric acid and dehydrated aluminumhydroxide fall through air into a column containing hydrocarbon andammonia phases. The droplets assume spheroidal shapes in passing throughthe water-immiscible liquid and then are coagulated to firm spheroidalbeads or pellets in the coagulating medium. Similar processes utilizingpseudosol feeds and hydrochloric acid are described in:

1. Katsobashvili et al., "Formation of Spherical Alumina and AluminumOxide Catalysts by the Hydrocarbon-Ammonia Process--1. The Role ofElectrolytes in the Formation Process", Kolloidnyi Zhurnal, Vol. 28, No.1, pp. 46-50, January-February, 1966;

2. Katsobashvili et al, "Preparation of Mechanically Strong Alumina andAluminum Oxide Catalysts in the Form of Spherical Granules by theHydrocarbon-Ammonia Forming Method," Zhurnal Prikladnoi Khimii, Vol. 39,No. 11, pp. 2424-2429, November, 1966; and

3. Katsobashvili et al., "Formation of Spherical Alumina and AluminumOxide Catalysts by the Hydrocarbon-Ammonia Process--CoagulationalStructure Formation During the Forming Process", Killoidnyi Zhurnal,Vol. 29, No. 4, pp. 503-508, July-August, 1967.

Catalysts are used to convert pollutants in automotive exhaust gases toless objectionable materials. Noble metals may be used as the principalcatalytic components or may be present in small amounts to promote theactivity of base metal systems. U.S. Pat. Nos. 3,189,563 to Hauel and3,932,309 to Graham et al. show the use of noble metal catalysts for thecontrol of automotive exhaust emissions. U.S. Pat. No. 3,455,843 toBriggs et al. is typical of a base metal catalyst system promoted withnoble metal. Unpromoted base metal catalysts have been described in U.S.Pat. No. 3,322,491 by Barrett et al.

The activity and durability of an automotive exhaust catalyst is in partdependent on the location and distribution of noble metals on thesupport. Since the use of noble metal is controlled to a great extent bycost, small amounts of noble metals should be placed on the support in amanner that achieves the best overall performance over the life of thecatalyst.

Several competing phenomena are involved in the surface treatment.Impregnating the maximum amount of the support particle provides thegreatest amount of impregnated surface area. However, since gasvelocities are high and contact times are short in an automotive exhaustsystem, the rate of oxidation of carbon monoxide and hydrocarbons andthe reduction of nitrogen oxides are diffusion controlled. Thus, thedepth of impregnation should not exceed the distance that reactants caneffectively diffuse into the pore structure of the particle. A balanceof impregnated surface area coupled with proper dispersion andaccessibility should be achieved to formulate a practical catalyst.

Catalytic metal accessibility and dispersion will provide initial highcatalytic activity, once the catalyst reaches operating temperature.However, since significantly high amounts of hydrocarbons, carbonmonoxide, and other partially combusted materials are produced inexhaust gases during the initial moments of the engine start, thecatalyst should have low thermal inertia in order to operate efficientlywhen the reaction zone is at a relatively low temperature.

A common deficiency of exhaust catalysts is decreased activity whenexposed to high temperatures, mechanical vibration and poisons presentin the exhaust such as lead, phosphorus, sulfur compounds, etc., forlong periods of use of up to 50,000 miles or so. An effective catalystwill retain its activity through resistance to noble metal crystallitegrowth, poisons, crystalline phase changes, and physical degradation.

An optimum high temperature alumina catalyst support has low density andhigh macroporosity while retaining substantial surface area and crushstrength and attrition resistance. Furthermore, it is stable incrystalline phases and geometric volume occupied. Difficulties have beenencountered in achieving the proper balance of these interrelated andsometimes competing properties and in combining an alumina support andmetal impregnation techniques to provide a catalytic converter capableof decreasing automotive exhaust emissions to the levels required bypresent and further government standards.

According to the present invention, an alumina composition havingproperties uniquely suitable for the formation of spheroidal particlesis prepared by a process in which the alumina is precipitated underspecific and controlled reactant concentrations and reactiontemperature, time, and pH and aged at a higher pH subsequent tofiltration. In the process, a sufficient amount of an aqueous solutionof aluminum sulfate having an Al₂ O₃ concentration of about 5 to about 9weight percent and a temperature of about 130° to about 160° F. is addedto water at a temperature of about 140° to about 170° F. to adjust thepH of the mixture to 2 to 5. An aqueous solution of sodium aluminatehaving an Al₂ O₃ concentration of about 18 to about 22 weight percentand a temperature of about 130° to about 160° F. and a further amount ofaqueous aluminum sulfate solution are simultaneously added to themixture to precipitate alumina and form an alumina slurry. The pH andtemperature of the slurry are maintained during the precipitation fromabout 7 to about 8 and from about 140° to about 180° F. respectively,and a rate of addition of the solutions is maintained to formintermediate boehmite--pseudoboehmite alumina. The pH of the slurry isthen adjusted to about 9.5 to about 10.5. The slurry is then filteredand the filter cake washed to provide a substantially pure alumina. Theprocess is reproducible and prepares a hydrous alumina from whichprocess impurities can be easily removed by water washing andfiltration. The control of temperature, time, rates, concentrations andpH produces an alumina which is a substantially pure, microcrystallinepsuedoboehmite-boehmite intermediate having from about 70 to about 85weight percent of the total amount of Al₂ O₃ present in crystallineform.

Substantially uniform spheroidal alumina particles having an unexpectedcombination of low density and high surface area, macroporosity, phasestability, and mechanical strength are prepared, preferably from the wetor dried boehmite--pseudoboehmite intermediate, by the improved externalgalation process of this invention. A slurry of alumina is prepared inan acidic aqueous medium and droplets of the slurry are passed throughair into a column containing an upper body of a water-immiscible liquidand ammonia and a lower body of aqueous alkaline coagulating agent. Theresulting spheroidal particles are aged in aqueous ammonia to thedesired hardness. The aged particles are dried and calcined.

It has also been discovered that a catalyst comprising a catalyticallyactive metal or metal compound impregnated on the spheroidal aluminaparticles has excellent activity and durability in many catalyticsystems. It is especially suited for eliminating pollutants inautomotive exhaust streams because of its quick light off and sustainedactivity under high temperatures and mechanical vibrations present inexhaust systems.

The process for preparing alumina of this invention comprises five mainphases. Phase I involves the formation of boehmite crystal seeds atacidic pH's in very dilute aqueous systems and is referred to as thenucleation phase.

Phase II, which is the main phase, involves the precipitation andcrystallization of alumina at a pH from about 7 to about 8. During thisphase, crystallites of boehmite or pseudoboehmite grow from the hydrousprecipitated alumina onto the crystalline seeds. Phase II is called theprecipitation and crystallite growth phase.

Phase III involves changing the pH of the system by the addition of analkaline solution in order to reduce the electrical surface charge onthe alumina precipitate. During this phase, the positive charge of thealumina particles is gradually reduced until at pH 9.4-9.6, it becomesessentially zero. In this condition, the alumina precipitate is said toexist at its isoelectric point. That is the point in pH at which thesurface does not exhibit any electrical charge. Phase III is, therefore,called the surface electrical charge reduction phase.

Optional Phase IV involves the aging of the system for predeterminedperiods of time, and phase V involves the filtering and washing of theresulting slurry in order to remove undesirable electrolytes orimpurities.

An optional final step of the process is the drying of the washed filtercake to a powdery material. This may be done with or without theincorporation of specific additives in order to reduce the absorption ofimpurities.

The reactants used to carry out the process of this invention are watersoluble aluminum salts, such as aluminum sulfate, aluminum nitrate,aluminum chloride, and the like; and an alkali metal aluminate, such assodium aluminate, potassium aluminate, and the like. In specificembodiments, the preferred reactants are aluminum sulfate and sodiumaluminate for reasons of cost, availability, and desirable properties incarrying out the invention. The reactants are used in the form ofaqueous solutions. The aluminum sulfate may be used over a wide range ofconcentrations above about 5 weight percent; however, for practicalreasons, it is used preferably in high concentrations from about 6 toabout 8 wt.% equivalent Al₂ O₃.

The sodium aluminate solution should be a relatively freshly preparedsolution exhibiting no precipitated or undissolved alumina. The sodiumaluminate may be characterized by its purity and equivalent aluminaconcentration, which should be in excess of about 16 weight percent,preferably about 18 to 22 weight percent equivalent Al₂ O₃. Furthermore,it should contain enough alkali, such as equivalent Na₂ O, to assurecomplete dissolution of the alumina. The sodium aluminate should exhibitan Na₂ O to Al₂ O₃ mole ratio in excess of about 1.2, preferably inexcess of about 1.35. Of course, for economic and practical reasons, theupper limit of the mole ratio should not be too great so that inpractical commercial processes, the ratio will not exceed about 1.5.Impurities, insufficient levels of soda, and high dilutions, will makethe sodium aluminate unstable.

Before the start of the process, the reactant solutions are heated to atemperature of about 130 to about 160° F., preferably to about 140° F.The reaction starts with the nucleation phase in which an initial chargeor heel of deionized water is placed in a suitable reactor tank. Thewater is agitated and heated to a temperature from about 140° F. toabout 170° F. In general, the temperature of the heel is anywhere fromabout 5 to about 10° F. below the target temperature at which thereaction is to be run.

An initial charge of aluminum sulfate is added to the water in a verysmall amount sufficient to adjust the pH of the mixture to a valuebetween about 2 and about 5, preferably between about 3 and about 4. Atthis point, the concentration of equivalent alumina in the mixtureshould not exceed about 0.1 weight percent, preferably about 0.05 weightpercent. The combination of very low concentration, low pH, and hightemperature results in the partial hydrolysis of the aluminum sulfatewith the concomitant formation of extremely small crystallites ofboehmite. This nucleation process takes place rapidly and the beginningof phase II may start soon after the first addition of aluminum sulfate.However, it is preferred to wait up to about 10 minutes, preferablyabout 5 minutes, in order to insure that the nucleation phase has runits course and the system has been properly seeded.

The second phase in the process is carried out by simultaneously addingthe sodium aluminate and aluminum sulfate reactants to the mixture thatcomprises the water heel containing the crystalline seeds. Thesesolutions are added simultaneously from separate streams into thereactor at preset and essentially constant rates to precipitate thealumina and form an alumina slurry. However, since the reaction is to becarried out at a pH between about 7 and about 8, the rate of addition ofone of the reactants may be slightly adjusted during the run to insurethat the desired pH range of the slurry is reached rapidly.

During this phase, the pH will rapidly climb from about 2 to about 5 toabout 7 to about 8. As the reactants are added to the initial heel, thealumina concentration in the resulting slurry will gradually increase.Under the conditions specified, the precipitate of alumina will tend tocrystallize into an intermediate boehmite--pseudoboehmite crystallinealumina. If the rate of precipitation exceeds the rate ofcrystallization for the particular conditions used, the excess hydrousalumina will remain in the precipitate as alumina gel. The aluminaprepared in the present invention exhibits a balance between the amountof crystalline boehmite--pseudoboehmite intermediate and the amount ofgel. This requires that the rate of precipitation as dependent on therate of addition of the reactants, exceeds by a controlled amount therate of crystallization of the hydrous alumina into theboehmite--pseudoboehmite intermediate. The pH and temperature of theslurry and the rates of addition of the reactants are maintained duringthe precipitation to form crystalline boehmite--pseudoboehmiteintermediate. Since the rate of crystallization is principally set bythe temperature of the system, the rates of addition of the reactantswill vary depending on the particular temperature at which the reactionis carried out. On the low temperature side of the operable temperaturerange, the rate of crystallization will be relatively slow and,consequently, the addition of the reactants should proceed at a slowrate. Generally, the temperature is maintained from about 140 to about180° F. For example, for temperatures in the range of about 140° toabout 150° F., the rate of addition should make phase II last in excessof 60 minutes, preferably in excess of 70 minutes.

On the other hand, on the upper side of the operable temperature range,such as about 170° to about 180° F., the rate of addition can bemarkedly increased so as to carry out phase II in shorter times, such asabout 15 to about 30 minutes. In the preferred precipitation reaction,the temperature will be between about 150° and about 170° F. and therate of precipitation should be controlled so as to carry the reactionover a period of about 30 minutes to about 70 minutes, preferably fromabout 40 minutes to about 60 minutes.

In a very specific embodiment, the reaction should be carried out at155° to 165° F. with the addition of the reactants carried out over aperiod of about 48 to about 52 minutes by maintaining the flow ofreactants essentially constant over the entire precipitation phase andadjusting the relative flows, if necessary, to provide a programmed pHas follows:

    ______________________________________                                        Time in Minutes                                                               From Start of      pH Range                                                   Phase II           Of Slurry                                                  ______________________________________                                         5 ± 2          7.0 -7.4                                                   10 ± 2          7.2-7.5                                                    20 ± 2          7.3-7.5                                                    50 ± 2          7.35-7.45                                                  ______________________________________                                    

In general, the exothermic nature of the reaction will providesufficient heat to maintain the temperature at the desired levelprovided the water heel and reactants are preheated as prescribed. Inthe event that difficulty is experienced in maintaining a desiredtemperature, external cooling or heating may be provided to insureappropriate temperature control. Carrying out the precipitation reactionunder the prescribed conditions will insure the formation of aprecipitated alumina exhibiting the desired balance between crystallinealpha alumina monohydrate (boehmite--pseudoboehmite intermediate) andgel.

At the end of phase II, the concentration of equivalent Al₂ O₃ in theslurry should range from about 5 to about 9 weight percent, preferablyfrom about 6 to about 8 weight percent. Material balance of thereactants added to this point may be carried out which will indicatethat the ratio of sodium expressed as moles of Na₂ O to sulfate ionexpressed as moles of equivalent H₂ SO₄ will be in excess of 0.80,preferably in excess of 0.88, but below 0.97.

At the conclusion of phase II, when the desired quantity of alumina hasbeen precipitated, the flow of aluminum sulfate is stopped. Phase III isthen conducted in order to reduce the electrical charge on the surfaceof the precipitated alumina from a strong positive level to zero orpossibly to a low negative value. This is done by adjusting the pH fromthe value at the end of phase II of about 7 to about 8 to a value nearor slightly in excess of the isoelectric point for alumina, which issomewhere between 9.4 and 9.6. Generally, the pH of the slurry isadjusted to about 9.5 to about 10.5, preferably about 9.6 to about 10.0.

The pH change may be done through the use of any strong alkalinesolution, such as sodium hydroxide. However, from a practical point ofview, it is desirable to continue using as the alkaline solution theinitial sodium aluminate reactant solution. By doing so, while the pHchange takes place, an additional amount of alumina will be incorporatedinto the slurry to increase yields and reduce costs. Consequently, in apreferred embodiment of the present invention, phase III is carried outby the continued addition of sodium aluminate at reduced rates. Thisinsures that the pH target is not exceeded or that localizedover-concentrations of the sodium aluminate will not cause theprecipitation of alumina under high pH conditions favoring the formationof undesirable crystalline phases, such as bayerite (beta aluminatrihydrate). During phase III, the temperature of the slurry ismaintained and the agitation is continued as during phases I and II. Thesodium aluminate is added at a slow rate first continuously until the pHis raised to about 9.5 to about 10.0 and then discontinuously, ifnecessary to reach the final pH target. During this phase, the sulfateion which is fixed onto the surface of the alumina particles is freed asthe positive charge of the alumina surface is reduced to zero or madesomewhat negative. When a pH from 9.5 to 10.5 is obtained, preferably apH of about 9.6 to about 10.0, the flow of sodium aluminate isdiscontinued and phase III may range from as low as 4 minutes to as longas 20 minutes, preferably it should be carried out over a period of timefrom about 6 to about 12 minutes.

In phase IV, after all the reactants have been added, the slurry may beaged up to several hours depending on practical considerations such asthe readiness of the equipment used in the next steps of filtration andwashing. In general, aging for about 30 minutes is used. In any event,since a large batch of alumina slurry prepared through phase III may notbe filtered in a single batch, but may be done continuously over aperiod of time, some of the material will, of necessity, be aged whilewaiting for the filtration step.

The alumina slurry is then filtered and the filter cake washed toeliminate undesirable impurities. Preferably, deionized water is used asthe washing liquid to remove water-soluble impurities. The use ofwatercontaining impurities, for example, calcium, magnesium, chloride,carbonate or bicarbonate, is undesirable. However, depending on theultimate use of the alumina, some of these impurities such as thevolatile impurities may be tolerated. However metal impurities such ascalcium, magnesium, iron, silicon, nickel, etc., cannot be toleratedexcept in extremely small concentrations.

The term "substantially pure" as used in this specification and claimsrefers to alumina having levels of impurities expressed on a dry basisthat do not exceed the following limits: Sodium expressed as Na₂ O, 0.15weight percent; calcium expressed as CaO, 0.15 weight percent; magnesiumexpressed as MgO, 0.15 weight percent; silicon expressed as SiO₂, 0.80weight percent; iron expressed as Fe₂ O₃, 0.07 weight percent; nickelexpressed as NiO, 0.07 weight percent.

The use of deionized water will result in a pure alumina product. Thedeionized water is usually heated in order to facilitate the removal ofelectrolytes and may have a temperature of 120 to about 180° F. Theremoval of electrolytes during washing is facilitated by the crystallinenature of the precipitate. Crystalline materials exhibit a lowertendency to occlude impurities and are easier to wash because ofimproved filtration characteristics. The amount of deionized waterrequired to achieve good quality in the product may vary depending onthe particular filtration equipment used. However, it will normallyrange from about 20 lbs. to 100 lbs. of water per pound of Al₂ O₃ (drybasis) in the filter cake.

In certain specific embodiments, the washed filter cake can be used toprepare a satisfactory feed for the preparation of alumina spheroids asdescribed in greater detail hereinafter. This can be done by removing asmuch water as possible during the filtration step and using thede-watered filter cake together with specific additives for thepreparation of the feed.

In a preferred embodiment, the filter cake is dried to produce a powderof alumina which can be conveniently stored without degrading for longperiods of time prior to use in further processing. The drying of thefilter cake may be done by several methods, such as tray drying, beltdrying, and the like. However, the preferred methods involve theaddition of water to the filter cake in order to form a pumpable slurryand the quick removal of water by such methods as spray drying or flashdrying. In these cases, the pumpable alumina slurry contains about 15 toabout 20 weight percent solids. The slurry is delivered through a nozzleinto the drying chamber as finely divided droplets. The droplets comeinto contact with hot drying gases. For example, in the spray dryer theinlet temperature of the drying gases ranges from about 800° to about700° F. The rate of addition of slurry is adjusted so as to obtain anexit temperature of greater than about 250° F., but not to exceed about400° F., preferably between about 300° and about 350° F. The use ofthese conditions insures the partial removal of water without destroyingthe crystalline nature of the alumina.

In the case where the drying gases are the combustion products of thefuel used, these gases contain substantial concentrations of carbondioxide. Carbon dioxide upon coming in contact with the alkaline slurrywill be absorbed and possibly chemically reacted on the surface. In suchcases, the end product will contain a small amount of carbon dioxide asan impurity. This carbon dioxide is not of major concern in most of thesteps which follow. However, if conditions so demand, it is possible toreduce the pick-up of carbon dioxide by previously acidifying thepumpable slurry with a trace amount of an acid to a pH of about 7 orlower. Preferred acids in this step are thermally decomposable organicacids, such as acetic and formic acid. Mineral acids may become fixedimpurities which will affect the process at a later stage; or, in thecase of nitric acid, be a source of undesirable pollutants.

During spray drying, a portion of the gel content of the precipitate maycrystallize to the boehmite--pseudoboehmite intermediate depending onspecific conditions used. The spray dried product is not completely drybut contains a certain proportion of water. In general, the spray driedproduct will contain at least about 18 weight percent water and mayrange upwards to about 33 weight percent water. Preferably, the range ofwater content will be between about 20 and about 28 weight percent.

Alumina as used herein refers to an alumina material containing Al₂ O₃,water of hydration, associated water, and the like. The degree of dryingof the alumina may be expressed in terms of the weight percent of Al₃ O₃therein. Drying at 1200° C. for 3 hours is considered to produce 100%Al₂ O₃.

The partially dried, hydrous alumina produced by the controlled reactionof sodium aluminate and aluminum sulfate is an intermediate betweenboehmite and pseudoboehmite. This form of alumina is alpha aluminamonohydrate with extra water molecules occluded within the crystalstructure and has the formula Al₂ O₃.xh₂ O where x is a value greaterthan 1 and less than 2. The boehmite--pseudoboehmite nature of theproduct, including its crystalline structure, the degree ofcrystallinity and average size of the individual crystallites, may bedetermined by X-ray diffraction techniques.

Pseudoboehmite is discussed by Papee, Tertian and Biais in their paper:"Recherches sur la Constitution des Gels et des Hydrates Cristallisesd'Alumine," published in the Bulletin de la Societe Chimique de France1958, pp. 1301-1310.

Boehmite is a well-defined mineral known for many years whosecrystalline nature and X-ray diffraction pattern are given in ASTM cardNo. 5-0190.

Other properties which characterize the product of our invention are:its behavior during aging under alkaline conditions; its ability tochemisorb anions such as sulfates at various pH's; its crystallinenature and stability after severe thermal treatments; and the hightemperature stability of its surface area, pore volume, and pore sizedistribution.

All of the above properties stem from the unique balance of crystallineand amorphous gel components in the product combined with its excellentoverall chemical purity.

The X-ray diffraction technique employed to determine the degree ofcrystallinity is as follows: the X-ray diffraction pattern of theproduct under study is determined using any of the several X-raydiffraction units commercially available, such as a Norelco X-raydiffractometer. A pattern is obtained which gives the location andintensity of the diffraction peaks. This pattern is compared with thedata given in ASTM card No. 5-0190 on boehmite. A matching of all of thediffraction peaks indicates that the product is boehmite. However, ifthe [020] peak is shifted to 6.6-6.7 A while the other peaks remainessentially unchanged this indicates the presence of pseudoboehmite. Thenature of a product can be further defined by determining the exactposition of the [020]0 d-spacing. Intermediate values of 6.2 to 6.5 Aindicate the presence of materials of intermediate nature. The [020]peak can further be used to determine the degree of crystallinity of thematerial. The area under this diffraction peak is measured with aplanimeter and compared with the area under the corresponding peak of areference sample run under identical conditions in the X-raydiffractometer. The reference sample is selected from a product known tohave a high proportion of boehmite and defined as "100% boehmite". Theratio of the areas provides a relative measure of the degree ofcrystallinity of the sample under study.

Finally, the nature of the crystallinity can be further detailed bymeasuring the width of the [020] diffraction peak. Mathematicalrelationships have been derived by others and published in theliterature which allow the calculation of the average crystallite sizein A as a function of the peak width measured half way of the maximumpeak intensity.

For a particular diffraction peak, the average crystallite size isinversely related to the width of the peak at half its maximumintensity. Relative measurements can be made by simply measuring thewidth at half the maximum intensity of the [020] diffraction peak. Largevalues of the width correspond to small crystallite sizes while smallvalues of the width correspond to large crystallite size. For example, a100% crystalline alpha alumina monohydrate obtained by the dehydrationof well-defined large crystals of alpha alumina trihydrate gives verytall and narrow diffraction peaks. In this material, the [020]reflection occurs at 6.1 A indicating that the product is boehmite asopposed to pseudoboehmite, and the width at half maximum intensity isonly about 0.2 A indicating the presence of large crystallites.

In contrast, microcrystalline pseudoboehmite exhibits the [020] peak atvalues ranging from 6.6 to 6.7 A. These materials show much wider peakswith values at half maximum intensity of about 2 A or greater. In otherwords, these materials exhibit a crystallite size approximately oneorder of magnitude greater than the 100% crystalline alpha aluminamonohydrate.

The product of our invention exhibits an intermediateboehmite--pseudoboehmite structure characterized by a [020] d-spacingwhich ranges from about 6.2 to about 6.5 A, preferably from about 6.3 toabout 6.4 A. The half maximum intensity width of the [020] peak rangesfron about 1.65 to about 1.85 A, preferably from about 1.75 to about1.80 A.

In terms of relative crystallinity, our product exhibits values fromabout 70 to about 85 weight percent of the total amount of Al₂ O₃present in crystalline form. The boehmite--pseudoboehmite product of ourinvention is characterized by high crystalline purity, by smallcrystallite size--i.e., microcrystallinity and by a high relative degreeof crystallinity. In these respects, the product is unique by virtue ofthe fact that it is prepared under conditions which give a high ratio ofcrystalline material to amorphous gel. This is in contrast withmaterials of the prior art in which the fraction of amorphous gel in theproduct is either quite high or essentially non-existent such as inboehmite. The intermediate nature of the crystallinity in our materialmakes it unique in its application as a starting powder for thepreparation of catalytic supports of excellent and unexpectedproperties.

The nature of the balance between crystalline and amorphous materials inour product may be further characterized by the following tests:

o The conversion of gel components to undesirable crystalline phases,such as bayerite, and

o Anion surface chemisorption at different pH's.

Amorphous hydrous aluminas have a tendency to crystallize. Theparticular crystalline phase which is obtained depends on the nature ofthe environment around the alumina during crystallization. A materialconsisting of boehmite or pseudoboehmite and containing high proportionsof gel components will crystallize to beta trihydrate (bayerite) ifexposed to elevated temperatures for long periods of time in an alkalineaqueous environment. in contrast, materials containing little or no gelcomponents will not develop the bayerite crystalline phase under similarconditions of alkaline aging. For example, an alumina prepared at lowtemperatures and consisting principally of pseudoboehmite interdispersedwith a high proportion of gel will, upon aging at least for about 18hours at about 120° F. in a sodium hydroxide aqueous solution of a highpH such as 10, develop bayerite while otherwise remaining essentiallyunchanged in its crystalline nature. This indicates that the formationof the bayerite is not at the expense or disappearance of pseudoboehmitebut that it is formed from the amorphous alumina gel. In contrast, theproduct of our invention treated under the same conditions will notexhibit the presence of any bayerite. This indicates that the amount ofgel in our material is quite small or otherwise more stable.

The anion chemisorption tests involve the preparation of a slurry of thealumina powder to be studied with deionized water, and thepotentiometric titration of this slurry with dilute sulfuric acid ofknown normality over a pH range in which alumina is insoluble. Thetitration is carried out slowly to make sure that there is ample timefor the acid to diffuse into the structure of the alumina product. Overthe pH ranges in question from about 9 to about 4 alumina is insoluble,so the titration with sulfuric acid is regarded as a measure of theamount of sulfate which becomes fixed or chemisorbed on the surface ofthe alumina at a given pH. For different aluminas, the amount of acidrequired to reach a particular pH from a common starting point, is anindirect measure of the extent of the alumina interface surface exposedto the aqueous medium. Materials which exhibit a very high degree ofcrystallinity and very large crystallite size possess a small interfacesurface area and consequently, require small amounts of acid to effect agiven change in pH. In contrast, materials which are very high in gelcontent exhibit high interface surface areas and, consequently, requirelarge amounts of acid to effect the same pH change. Products ofintermediate crystalline/gel nature will require intermediate amounts ofacid to effect the same pH change.

For example, 100% crystalline alpha alumina monohydrate which consistsof very well-defined large crystallites requires only about 53milliequivalents of sulfuric acid per mole of alumina to change the pHfrom an initial value of about 8.3 to a final value of about 4.0. Incontrast, an alumina prepared at low temperatures in which thepseudoboehmite nature, percent crystallinity and crystallite sizeindicate a low degree of crystallinity and a high gel content, requiresabout 219 milliequivalents of sulfuric acid per mole of alumina toeffect the same change in pH.

The composition of our invention is characterized by intermediaterequirements of sulfuric acid to effect the pH change. From about 130 toabout 180 milliequivalents of sulfuric acid per mole of alumina,preferably from about 140 to about 160 milliequivalents, will change thepH of a slurry of our composition from about 8.3 to about 4.0.

The application of the alumina powder product of our invention in makingsuitable supports for automotive exhaust catalysts requires that thematerial exhibit good stability of its structural properties at elevatedtemperatures. For example, its pore volume and surface area, determinedafter severe thermal treatments simulating those which a catalystencounters during use, should remain high and stable. These hightemperature properties are highly dependent on the purity of the initialmaterial and its structural features as well as the crystalline natureof the product after thermal treatments.

Our material will upon heating lose gradually its water of hydration andother associated or bound water. This dehydration will cause atransition of the crystalline structure to gamma alumina. Furtherheating to higher temperatures will cause the gamma alumina to convertto delta and eventually to theta alumina. All of these aluminas aretransition aluminas of high surface area and pore volume. Heating tostill higher temperatures will cause the formation of alpha alumina orcorundum which is not a transition alumina and exhibits a very lowsurface area and pore volume. The final transition to alpha alumina isso profound that its formation is accompanied by dramatic decreases inpore volume and surface area. A good alumina powder capable ofconversion to good automotive exhaust catalyst supports should bethermally stable and not exhibit the transition to alpha alumina atmoderately high temperatures, such as 1800°-1900° F. In general,aluminas with high gel content will have a tendency to sinter to alphaalumina at relatively moderate temperatures, such as 1800°-1900° F.Materials which have been prepared at low temperatures and which exhibithigh gel content as measured by several of the tests given in thisspecification, will show the appearance of undesirable alpha aluminawhen heated, for example, to 1850° F. for one hour.

In contrast, the product of our invention, which contains only a smallamount of amorphous gel, will remain stable and will not show any alphaalumina under identical thermal treatment. Our composition has anX-diffraction pattern of theta alumina, gamma alumina, and delta aluminaafter heating at about 1850° F. for about one hour. Furthermore, theproduct of our invention will retain at those temperatures verysubstantial surface areas and pore volumes which will remain stable evenfor prolonged periods of time under severe thermal treatments.

Our product after a thermal treatment of about 1 hour at about 1850° F.,will exhibit a BET nitrogen surface area of about 100 to about 150square meters per gram, more commonly of about 110 to about 140 squaremeters per gram. It will also exhibit a nitrogen pore volume from about0.60 to about 0.75 cm.³ /g., most commonly from about 0.64 to about 0.72cm.³ /g.

Furthermore, the pore structure of this thermally treated material willnot exhibit a high proportion of microporosity as determined by nitrogenpore size distribution methods. Typically, our product will not exhibitany nitrogen pore volume below about 80 A size, more commonly below 100A.

Throughout this specification and claims, the "nitrogen pore volume"refers to the pore volume as measured by the techniques described in thearticle by S. Brunauer, P. Emmett, and E. Teller, J. Am. Chem. Soc.,Vol. 60, p. 309 (1938). This method depends on the condensation ofnitrogen into the pores, and is effective for measuring pores with porediameters in the range of 10 to 600 A.

The surface areas referred to throughout this specification and claimsare the nitrogen BET surface areas determined by the method alsodescribed in the Brunauer, Emmett, and Teller article. The volume ofnitrogen adsorbed is related to the surface area per unit weight of thesupport.

Dried alumina powders or washed alumina filter cake with the propercrystalline character as prepared by this invention are preferably usedin preparing the feed for the oil-drop forming process. However, othersuitable starting alumina compositions as described hereinafter may alsobe used to form spheroidal alumina particles in our improved process.The alumina and an acidic aqueous medium, such as an aqueous solution ofan acid or acid salt, are commingled to provide a slurry. Preferably, anaqueous solution of a monobasic mineral acid is commingled with waterand the alumina to provide the slurry. Use of a monobasic acid providesa homogeneous, plastic slurry with the desired viscosity. Hydrochloricacid and other strong monobasic acids may be used and the support washedfree of these electrolytes. Aluminum nitrate may be used. Nitric acid ispreferred because it is decomposed and removed from the spheroids byheating later in the process so that washing the spheres is notnecessary. In order to minimize the nitrogen oxides produced in thelater states as noxious emissions, a decomposable monobasic organic acidsuch as acetic acid, (hereinafter represented symbolically as CH₃ COOH),formic acid, or mixtures thereof, preferably replaces a major portion ofthe nitric acid. For example, a mixture of organic acid and nitric acidin a molar ratio of about 0.5 to 5 may be employed.

Bulk density and crush strength of the spheroid product depend upon feedcomposition. Increasing alumina and/or acid content of the feedincreases these physical properties. Too high a concentration of aluminaand/or acid may result in spheroid fracture upon drying and too low aconcentration in weak, powdery spheroids. Because of the gel content ofthe alumina powder used in preparing the feed, a minor amount of acid issufficient to form a plastic slurry. The slurry may contain about 1 toabout 12 weight percent of a monobasic acid or mixtures thereof and theslurry generally contains about 10 to about 40, preferably about 24 toabout 32 weight percent of alumina and has a molar ratio of acid toalumina of about 0.05 to about 0.50. The quantity of water is sufficientto yield a slurry with these acid and alumina contents. Normalizing thesystem in relation to one mole of alumina, the inorganic acid molarratio may vary between 0.5 to 0.03, preferably 0.06, and the organicacid molar ratio from 0 to 0.03, preferably 0.12, and the water molarratio may be about 5 to about 50, preferably about 10 to about 20. Anespecially preferred slurry has a molar composition of

    (Al.sub.2 O.sub.3).sub.1.00 (CH.sub.3 COOH).sub.0.12 (HNO.sub.3).sub.0.06 (H.sub.2 O).sub.14.0±1.5

The slurry may be prepared from a single alumina composition or a blendof alumina compositions. Blends are used to take advantage of somespecific properties of the individual components of the blend. Forexample, alumina filter cake may be acidified with acetic acid, to aboutpH 6.0, prior to spray drying to reduce carbon dioxide absorption. Ahigh carbonate content in the powders may result in sphere crackingduring drying. Thus, 20 parts of this low carbonate alumina may becombined with 80 parts of untreated dried powder to give a blend with anacceptable carbonate level. Preferably, the alumina powder and acidicaqueous medium are commingled in stages by adding portions of the powderto the medium to acidify the alumina and reduce the level of CO₂ thatmay be present in the spray dried alumina powder. For example, 80percent of the alumina required for a given batch of product may bemixed in water which contains the desired quantities of acid. After aperiod of mixing, the remaining 20 percent of the powder is then addedto the batch. In addition, recycled, calcined product fines in an amountof up to about 15 percent of total alumina may be added. This decreasesthe tendency of the product to shrink to about 2 to about 3 volumepercent. It also makes the process more economical in that scrap productsuch as fines, etc., can be recycled.

Agitation and aging of the slurry provide a uniform material with aviscosity that permits proper formation of the droplets from which thespheroids with low shrinkage can be made. Agitation of the slurry can beaccomplished by a variety of means ranging from simple hand stirring tomechanical high shear mixing. Slurry aging can range from a few minutesto many days. The aging time is inversely related to the energy inputduring mixing. Thus, the alumina powder can be stirred, by hand, intothe acid and water mix for 10 minutes and aged overnight to reach theproper consistency for droplet formation. For example, in a specificpreferred method using about 10 lbs. of powder, 60 percent of the powderis mixed with all of the acid and water and blended vigorously with a1/2 H.P. Cowles dissolver turning a 3 inch blade at about 3500 RPM. forabout 2 to about 30 minutes or preferably about 15 to about 20 minutes.The remaining 40 percent of the powder is then added and stirringrecommenced for about 5 to about 60 minutes and preferably about 30 toabout 40 minutes. After agitation, the slurry is aged for about 1 toabout 5 hours to reach the proper consistency. During mixing, the pHrises and a final pH of generally about 4.0 to about 4.8, preferablyabout 4.3 to about 4.4, is achieved. The viscosity of the slurry,measured immediately after the preferred blending technique, may varybetween about 60 and about 300 centipoises (cps). For optimum dropletformation, slurry viscosities of about 200 to about 1600 cps, preferablyabout 800 to about 1200 cps are desirable. Viscosities as high as 2000cps may be used but the slurries are difficult to pump.

Under actual operating conditions in a plant, there might be occasionsin which a slurry may have to wait for long periods of time prior tofurther processing. Under these conditions, the viscosity of the systemmay climb above the pumpable range. Such a thickened slurry need not bewasted. It still can be used by following any of the following twoprocedures:

The thick slurry may be diluted with controlled amounts of water andstrongly agitated for short periods of time. This will result in a sharpdecrease of the viscosity and will bring the system into the pumpablerange.

The thick slurry may be mixed with a freshly prepared slurry which willexhibit a low viscosity between about 60 and about 300 cps. Theresulting mixture will have a viscosity in the pumpable range and can beused in the process. Both of these remedial steps can be practicedwithout adversely affecting the properties of the finished product northe subsequent processing steps.

The viscosity of slurries referred to in these specifications, examplesand claims, is the viscosity as measured with a Brookfield viscometer.

The spheroidal particles are formed by gelation in an organic phase andan aqueous phase. Droplets of the aged slurry are formed in air above acolumn which contains an upper body of water-immiscible liquid andammonia and a lower body of an aqueous alkaline coagulating agent. Thedrops assume spheroidal shapes in passing through the upper phase andthen are coagulated into firm spheroical particles in the lower phase.The ammonia in the upper phase gels the droplet exterior layerssufficiently to allow the spheroidal shape to be retained as thedroplets cross the liquid interface and enter the lower phase. Excessiveinterfacial tension between the phases may result in retention of thedroplets in the organic phase and possibly their deformation. In suchcases, a small quantity of surfactant, for example, about 0.05 to about0.5, preferably 0.1 to about 0.2 volume percent of the upper body, isplaced at the interface and permits the spheres to penetrate it easily.Liquinox®, a detergent sold by Alconox, Inc., New York, N.Y., and othersuch surfactants may be employed.

The water-immiscible liquid will have a specific gravity lower thanwater, preferably lower than about 0.95, and can be, for example, any ofthe mineral oils or their mixtures. The organic liquid should not permitthe droplets to fall too rapidly which may inhibit proper sphereformation. Furthermore, it should not exhibit high interface surfacetension which may hold up and deform the particles. Examples of suitablemineral oils, include kerosene, toluene, heavy naptha, light gas oil,paraffin oil, and other lubricating oils, coal tar oils, and the like.Kerosene is preferred because it is inexpensive, commercially available,non-toxic and has a relatively high flash point.

The organic liquid should be capable of dissolving small amounts ofanhydrous gaseous ammonia or be capable of forming suspensionscontaining trace amounts of water which contain dissolved ammonia. Anessential requirement of the process is that the organic phase containssufficient, but small, amounts of a base, preferably ammonia, in orderto be able to effect the partial neutralization and gelation of theouter layers of the falling droplets. The rate of introduction ofammonia into the organic liquid should be sufficient to reach anoperating concentration in which firm particles will be formed in theshort time span of fall. However, the ammonia concentration should notbe so high as to cause essentially instantaneous gelation of the slurrydroplets as they enter the organic liquid. Under these conditions, thedroplets will gel into misformed particles since they have not hadsufficient time of fall to allow their surface tension to spheroidizethe droplet. Furthermore, high concentrations of ammonia in the upperregions of the organic liquid will cause evaporation of gaseous ammoniainto the air pocket where the nozzles are located. Excessive ammoniaconcentration in this region may cause premature gelation of thedroplets prior to the point of separation from the nozzle. This is veryundesirable because premature gelation in the nozzle will cause pluggingand malfunction of the delivery system. Ammonia is a preferredcoagulation agent because it produces good spheroids, exhibits aconvenient solubility, and may be conveniently introduced into the lowerportion of the organic liquid. In a preferred embodiment, the organicliquid is contacted with anhydrous gaseous ammonia in a separateapparatus called the ammoniator, and circulated through the column. Insuch an event, the organic liquid from the ammoniator is introduced inthe lower portion of the organic phase in the column and it flowsupwardly through the column establishing a counter current flow with thefalling droplets. The organic liquid is removed at the top of the columnand returned to the ammoniator for replenishing with added ammonia.

Under steady state conditions, an ammonia concentration gradientdevelops within the organic phase of the column. The gradient is causedby the reaction of the falling acidic alumina slurry droplets with theascending ammonia carried by the organic phase. Because of the lowerammonia concentration in the upper portions of the column, the dropletshave time to shape into spheroids before they gradually gel as theydescend. The ammonia concentration in the organic liquid may bedetermined by titration with hydrochloric acid to a bromthymol blueendpoint and may be maintained between about 0.01 to about 1.0,preferably about 0.04 to about 0.07, weight percent. Lowerconcentrations generally result in flattened spheroids, and higherconcentrations in deformations such as tail formation.

The length of the column can vary widely and will usually be from about3 to 20 feet in height. The organic phase may generally comprise about1/3 to about 2/3 of the column length and the coagulation phase theremainder.

The aqueous medium may contain any substance capable of inducinggelation and having an appropriate specific gravity, i.e. lower than thespecific gravity of the slurry droplets. This permits the spheres topass through it. Alkaline aqueous solutions such as sodium hydroxide,sodium carbonate, or ammonia can be used as the coagulating medium. Thepreferred medium is an aqueous solution of ammonia, because it and itsneutralization products are easily removed from the spheroids in laterprocessing steps. Washing it not necessary to remove the ammoniumresidue as it would be to remove a sodium residue. The ammoniaconcentration in the aqueous phase may be about 0.5 to 28.4 weightpercent preferably about 1.0 to about 4.0 weight percent. Duringprolonged use, ammonium nitrate and acetate may be formed and build upto steady state levels in the aqueous phase. These are products of theneutralization reaction occurring during sphere gelation. Their steadystate concentration will be dependent upon the concentrations of theacids in the alumina slurry feed. In the development of this inventionammonium acetate and ammonium nitrate were added to the aqueous ammoniaphase to simulate the effects of eventual steady state values of thesesalts. For the preferred slurry composition, the concentrations usedwere typically about 1.3 and about 0.8 weight percent respectively.

Under continuous operation, ammonia must also be constantly added to theaqueous phase to replace that used in gelation of the spheres. In apreferred embodiment of this invention, the aqueous phase is circulatedbetween the column and an ammoniator tank. This tank also serves as areservoir with a batch collection system to take up aqueous ammoniasolution displaced from the column as spheres fill up the collectionvessel. The aqueous phase is removed from the column to maintain aconstant interface level. In a continuous sphere take-off system, thereservoir feature of the aqueous phase ammoniator would not be needed.Either type of collection system can be used.

The cross sectional area of the column is dependent upon the number ofdroplet nozzles used. For one nozzle, a one inch diameter columnprovides approximately 5 cm.² of cross-sectional area, which issufficient to keep the uncoagulated droplets from hitting the columnwalls and smearing and sticking on the walls. A four inch diametercolumn provides enough cross-sectional area for up to about 16 to 20nozzles to permit the droplets to fall independently through the columnwithout contacting each other or the walls.

In one embodiment of a suitable column, the aged slurry is pumped into apressurized multiple orifice feed distributor that is located at the topof the oil column and contains a multiplicity of nozzles positionedabout 1/2 inch above the organic liquid. The pressure of the feeddistributor is dependent upon the slurry viscosity. Pressures of about0.1 to about 15 p.s.i.g. are normally used. The feed distributorpressure regulates the droplet formation rate. The latter varies fromabout 10 to about 250 droplets per minute with a preferred rate beingabout 140 to about 180 drops per minute. A distributor pressure of about1.5 to about 2.5 p.s.i.g. gives the desired droplet rate when the slurryviscosity is in the range of about 800 to about 1200 cps. The nozzlesemployed can vary in diameter to gives pheroidal particles of thedesired size. For example, a 0.11 inch internal diameter nozzle willproduce spheroids of a diameter of about 1/8 inch. Preferably, an airflow is provided around the nozzles to keep ammonia vapor fromprematurely gelling the droplets. The droplets of slurry are formed inair at the nozzle tips and fall through air into the body ofwater-immiscible liquid. When the drops of slurry initially contact theimmiscible liquid, they are usually lens-shaped. As the drops fallthrough ammonia-treated organic liquid, they gradually become spheroidalparticles which are set into this shape by the coagulating ammonia andharden further in the lower aqueous ammonia phase.

The particles are then aged in aqueous ammonia with a concentration ofabout 0.5 to 28.4 weight percent, preferably the same concentration asin the column. The particles develop additional hardness so that theyare not deformed during subsequent transfer and processing steps. Ingeneral, the particles may be aged from about 30 minutes to about 48hours, preferably about 1 to 3 hours.

The particles are then drained and dried. Forced draft drying to about210° to about 400° F. for about 2 to 4 hours may be advantageouslyemployed although other drying methods may also be used. In a preferreddrying method, the drying is done in a period of under 3 hours byprogramming the temperature to climb gradually and uniformly to about300° F. The amount of air used may normally vary between about 400 and600 standard cubic feet per pound of Al₂ O₃ contained in the wetspheroids. Under certain circumstances, some of the air may berecirculated in order to control the humidity of the drying medium. Thespheres are usually spread over a retaining perforated surface or screenat thicknesses ranging from 1 to 6 inches preferably 2 to 4 inches. Aslight shrinkage usually occurs during drying but the spheroids retaintheir shape and integrity.

Deviations from the prescribed conditions of preparation of starting rawmaterials may often result in significant changes in the productsobtained. Excessive powder particle size, crystallinity, or level ofimpurities may result in cracking and fracturing during drying. On theother hand excessive levels of gel in the powder, or pseudoboehmite mayresult in excessive shrinkage and densification upon drying which canalso lead to cracking. Alumina compositions other than the product ofour invention which are suitable for spheroid formation will generallyhave a boehmite or pseudoboehmite crystalline structure, preferablymicrocrystalline, a nitrogen pore volume of 0.4 to 0.6 cm.³ /g., surfaceareas in excess of 50 m.² /g. and will contain amorphous gel.

The dried spheroid product is then treated at high temperatures toconvert the crystalline alumina hydrate and amorphous gel components toa transition alumina. This may be done by batch or continuouscalcination by contacting the product with hot gases which may be eitherindirectly heated gases or the combustion products of ordinary fuelswith air. Regardless of the particular method used, the product iscalcined at specific temperature ranges depending on the particulartransition alumina desired.

For example, to obtain a gamma type alumina, the product may beconveniently calcined at temperatures of about 1000° F. to about 1500°F. For applications which require high temperature stability whileretaining high surface area and porosity, the target material may betheta alumina. A predominantly theta alumina product may be obtained bycalcination at about 1750° to about 1950° F. preferably about 1800° toabout 1900° F. for periods of from about 30 minutes to about 3 hours,preferably from about 1 hour to about 2 hours. For automotive exhaustcatalysts, the high temperature treatment step is often calledstabilization.

The catalyst support that comprises the spheroidal alumina particles andthat is obtained after stabilization has the following range ofproperties:

    ______________________________________                                                           Approximate General                                        Property           Range                                                      ______________________________________                                        Surface Area (m..sup.2 /g.)                                                                       80-135                                                    Compacted Bulk Density                                                        (lbs./ft..sup.3)   20-36                                                      Total Pore Volume (cm..sup.3 /g.)                                                                0.8-1.7                                                    Pore Size Distribution                                                        (cm..sup.3 /g.)                                                                Below 100 A         0-0.06                                                    100-1000 A        0.5-1.0                                                    1000-10,000 A      0.1-0.4                                                     Above 10,000 A      0-0.4                                                    Crush Strength (lbs.-Force)                                                                       5-15                                                      Volume Shrinkage (%)                                                                             0-6                                                        Attrition Loss (%) 0-5                                                        Mesh Size          -4 + 10                                                    ______________________________________                                    

However, when the preferred starting raw materials are used under thepreferred conditions of preparation, the property ranges become:

    ______________________________________                                        Property              Typical Range                                           ______________________________________                                        Surface Area (m..sup.2 /g.)                                                                          90-120                                                 Compacted Bulk Density (lbs./ft..sup.3)                                                             26-32                                                   Total Pore Volume (cm..sup.3 /g.)                                                                   0.9-1.2                                                 Pore Size Distribution (cm..sup.3.g.)                                          Below 100 A            0-0.04                                                 100-1000 A           0.6-0.9                                                  1000-10,000 A        0.2-0.3                                                  Above 10,000 A         0-0.3                                                 Crush Strength (lbs.-force)                                                                          7-12                                                   Volume Shrinkage (%)  2-4                                                     Attrition Loss (%)    0-2                                                     Mesh Size             -5 + 7                                                  ______________________________________                                    

The catalyst support may have a crush strength of at least about 5pounds or greater than about 7 pounds, a volume shrinkage of less thanabout 6% or less than about 4%, and an attrition loss of less than about5% or less than about 2%. The surface areas are nitrogen BET surfaceareas and the other above specified properties were determined by thefollowing methods. These methods may be also applied to the finishedcatalysts.

Compacted Bulk Density

A given weight of activated spheroids is placed in a graduated cylindersufficient to contain same within its graduated volume. "Activated" asused herein means treated at 320° F. in a forced draft oven for 16 hoursprior to the testing. This activation insures that all materials aretested under the same conditions. The cylinder is then vibrated untilall settling ceases and a constant volume is obtained. The weight ofsample occupying a unit volume is then calculated.

Total Specific Pore Volume

A given weight of activated spheroids is placed in a small container(for example, a vial). Using a micropipette filled with water, the saidsample is titrated with water until all of the pores are filled and theendpoint of titration occurs at incipient wetness of the surface. Thesemeasurements are consistent with total porosities calculated from theequation:

    P=(f/D)-(1/ρ)

in which:

P=total specific porosity (cm.³ /g.)

f=volume packing fraction (for spheroids typically 0.64±0.04)

D=compacted bulk density (g./cm.³)

ρ=crystal density of skeleton alumina (g./cm.³) (typically between 3.0and 3.3 g./cm.³ for transition aluminas)

Mercury Pore Size Distribution

The pore size distribution within the activated spheroidal particle isdetermined by mercury porosimetry. The mercury intrusion technique isbased on the principle that the smaller a given pore the greater will bethe mercury pressure required to force mercury into that pore. Thus, ifan evacuated sample is exposed to mercury and pressure is appliedincrementally with the reading of the mercury volume disappearance ateach increment, the pore size distribution can be determined. Therelationship between the pressure and the smallest pore through whichmercury will pass at the pressure is given by the equation:

    r=-2σCOS θ/P

where

r=the pore radius

σ=surface tension

θ=contact angle

P=pressure

Using pressures up to 60,000 p.s.i.g. and a contact angle of 140°, therange of pore diameters encompassed is 35-10,000 A.

Average Crush Strength

Crush strength is determined by placing the spheroidal particle betweentwo parallel plates of a testing machine such as the Pfizer HardnessTester Model TM141-33, manufactured by Charles Pfizer and Co., Inc., 630Flushing Avenue, Brooklyn, New York. The plates are slowly broughttogether by hand pressure. The amount of force required to crush theparticle is registered on a dial which has been calibrated in poundsforce. A sufficient number (for example, 50) of particles is crushed inorder to get a statistically significant estimate for the totalpopulation. The average is calculated from the individual results.

Shrinkage

A given amount of particles is placed in a graduated cylinder andvibrated until no further settling occurs, as is done in determiningCompacted Bulk Density. This sample is then placed in a muffle furnaceat 1800° F. for 24 hours. At the end of this exposure, its volume isagain measured after vibration until no further settling occurs. Theloss in volume after heating is calculated, based on the originalvolume, and reported as percent shrinkage.

Attrition Loss

A set volume (60 cc.) of material to be tested is placed in an invertedErlenmeyer flask of special construction which is connected to a metalorifice inlet. A large (one inch) outlet covered with 14-mesh screeningis located on the flat side (bottom) of the flask. High velocity drynitrogen gas is passed through the inlet orifice causing the particlesto: (1) circulate over one another thus causing attrition, and (2)impact themselves in the top section of the flask thus breaking down asa function of strength. The material is tested for five minutes and theremaining particles are weighed. The loss in weight after testingexpressed as percent of the initial charge is designated the attritionloss.

The nitrogen flow will be in the range of about 3.5 and 4.0 cubic feetper minute, depending upon the density of the material. The flow ratemust be sufficient for the particles to strike the top section of theflask. The fines produced by attrition are carried out of the flask bythe nitrogen flow thus causing a loss in weight of the original materialcharged.

The alumina and sphere formation conditions of the present inventionprovide spheroidal alumina particles with a highly unexpected anduniquely desirable combination of properties. The spheroids have a totalpore volume ranging from about 0.8 to about 1.7 cm.³ /g. While this is ahigh total pore volume, in itself it is not exceptional. What makes thispore volume exceptional is the size distribution of the pores which makeup this volume and high temperature stability of this volume. A largefraction of the volume is made up of macropores (>1000 A). Most of therest of the pores are in the 100-1000 A range. There are very fewmicropores (<100 A). This type of distribution is important forcatalytic activity and stability. In a heterogeneous process, catalyticactivity is highly dependent upon the rate of diffusion of reactants tothe catalyst sites and of reaction products away from the sites. Thus,reaction processes in a catalyst containing a large amount ofmacroporosity are less diffusion dependent. However, the macroporesaccount for only a small fraction of the sample surface area. Theintermediate size pores provide the surface area required for catalyticactivity. This surface area has two components; namely, that required bythe catalytically active clusters themselves and that required to keepthe clusters separated. If the clusters are allowed to fuse together,their catalytic surface area and consequently the catalyst activity willdecrease. Microporosity of course, provides a very large surface area,but, this does not necessarily provide good catalytic activity.Diffusion of reactants and/or products may be the rate controllingfactor. Micropores can be closed over by sintering occurring duringcatalyst operation or by deposition of poisons such as lead compounds inan auto exhaust system. In either case, the activity of the catalyst inthe closed micropores would be lost.

The surface area of the product spheroids is high, but is not unusuallyhigh. Surface areas range from about 350 m.² /g. to about 500 m.² /g.for spheroids heated to 1000° F. and drop to about 80 m.² /g. to about135 m.² /g. for thermally stabilized spheroids at 1800°-1900° F. What isimportant, however, is that most of the surface area is associated withintermediate size pores and not with micropores.

This preferred porosity distribution and its pore volume stability are adirect result of the unique combination of properties in the aluminapowder used to make the spheroids. In particular, its purity and highratio of crystalline material to amorphous gel aid in minimizingmicroporosity.

These properties also help to account for the high temperature stabilityof the spheroids. The spheroids exhibit low volume shrinkage, preferablyless than about 4%. They retain the transition alumina structure. Alphaalumina is not detected even at temperatures of 1950° F. It is wellknown that impurities act as sintering aids. Thus, high impurity levelscan promote shrinkage and alpha alumina formation. A high gel contentalso leads to alpha alumina formation at high temperatures. A highmicroporosity can result in high volume shrinkage as micropores areclosed off during sintering.

The spheroids also have an uncommon combination of low bulk density andrelatively high crush strength. Low bulk density is essential for quicklight off, i.e. high initial catalytic activity. The crystallinity ofthe alumina compositions is a contributing factor to both the low bulkdensity and high crush strength.

The low attrition loss exhibited by the spheroids is a directconsequence of their shape and strong structure. The smooth surface willnot attrit as readily as irregular surfaces which exhibit corners and/oredges. Also, the gelation process produces a coherent uniform particlerather than a layered particle which results from some mechanicalballing processes. A mechanically formed particle may delaminate duringan attrition process.

Another feature of this invention is the close control of the spheroidsize. In a given batch, greater than 95% of the spheres will be withinone mesh size, such as -5+6 or -5+7. Measurement with a micrometer showsthat the spheroids are even more closely sized. There is only about a0.015 inch variation in the major or minor axis of the spheroids. Thus,a controlled distribution of sphere sizes can be obtained by using theproper distribution of nozzle sizes. This can aid in controlling thepressure differential across a packed catalyst bed which is an importantfactor in auto emissions catalyst devices.

The properties of the spheroid product, when taken in toto define aunique particle which makes a superior catalyst support.

The support of this invention is characterized by low density, highdegree of macroporosity, high crush strength, high temperature shrinkresistance, good attrition resistance and controlled size and shape.

A catalyst comprising the support of this invention impregnated with acatalytically effective amount of at least one catalytically activemetal or metal compound is highly effective in many catalytic systemsparticularly those that operate at high temperatures. Although thealumina itself may be active as a catalyst, it is usually impregnatedwith a suitable catalytic material and activated to promote itsactivity. The selection of the catalytic material, its amount, and theimpregnation and activation procedures will depend on the nature of thereaction the catalyst is employed in. Preferably, the catalyticallyactive metal is platinum group metal selected from the group consistingof platinum, palladium, ruthenium, iridium, rhodium, osmium, andcombinations thereof.

In order to possess the high initial and sustained activity necessary tomeet the increasingly stringent emission controls required by state andfederal laws, the catalytic agents in a catalyst of this invention aredistributed in particular positions within the catalyst particle. Thecatalytic agents which are typically used in automobile exhaustcatalysts are the platinum group metals. Because of the great cost ofthese metals, it is uneconomical to use large quantities. Hence, it isimportant to position the metals in the most efficient as well as themost strategic manner. Suitable platinum group metals include, forexample, platinum, palladium, rhodium, ruthenium, iridium, and osmium,as well as combinations thereof.

The use of platinum group metals in automobile exhaust catalysis hasbeen constrained by the natural abundance of these metals. Since a largefraction of the world supply occurs in South Africa with the majormetals being platinum, palladium and rhodium which occur naturally inthe approximate proportions of 68 parts platinum, 27 parts palladium and5 parts rhodium, the majority of work has been with the platinum groupmetals in these proportions. Typically, the total noble metal content ofautomotive exhaust catalysts, based on the weight of the catalyst, isfrom about 0.005 to about 1.00 weight percent but preferably is fromabout 0.03 to 0.30 weight percent to be both economically as well astechnically feasible. Of course, the optimum combination of metals isthat which imparts specific performance benefits to the catalyst such asa higher level of palladium for more rapid oxidation of carbon monoxideand better thermal stability, or more platinum for greater poisonresistance and better durability of hydrocarbon oxidation, or increasedrhodium concentration for improved conversion of nitrogen oxides tonitrogen. Catalysts prepared according to this invention may containonly one of the platinum group metals, but usually contain several. Themetal ratios may vary over a wide range of values but preferably thecatalyst contains platinum and palladium in a weight ratio of about 5parts to about 2 parts, respectively, or platinum, palladium and rhodiumin a proportion of about 68 parts, 27 parts and 5 parts, respectively.

A variety of noble metal compounds are well-known and have beendocumented in the literature. The type of compound to be used dependslargely on the nature of the surface of the support and the resultinginteraction with it. For example, anionic or cationic forms of platinummay be introduced into the support by using H₂ PtCl₆ or Pt(NH₃)₄ (NO₃)₂,respectively.

The noble metal compound to be used should be introduced into thesupport such that once it is decomposed into the active form (metal ormetal oxide) it will be highly dispersed and positioned in a specificlocation in the catalyst particle. We have found that there arepreferred locations for the metals as well as preferred distribution ofone metal with respect to another for a high degree of initial activityand which is especially important for sustained durability. The abilityto control the location and dispersion depends on the noble metalcompound used. We have found that of particular utility are thosecomplexes described in U.S. Pat. No. 3,932,309 to Graham et al. In thismethod, the catalyst is prepared by impregnating the support withsulfite-treated platinum and palladium salt solutions. These sulfitocomplexes when applied to the support decompose to provide a high degreeof dispersion. By varying the cationic form of the complex, for example,NH₄ ⁺ or H⁺ form, the depth of impregnation of the metal within theparticle can be varied.

We have found that for high initial, and more importantly sustainedactivity, the noble metals should be positioned such that about 50% ofthe total noble metal surface area is deeper than about 50 microns. Withregard to the specific location of the noble metals, we have found thatthe preferred location of the noble metals should be such that about 50%of the active metals is located deeper than about 75 microns.Determination of noble metal surface area is carried out by the hydrogentitration of oxygen. The detailed method for carrying out thisdetermination is described by D. E. Mears and R. C. Hansford, J. ofCatalysis, Vol. 9, 125-134 (1967). Elemental analysis is carried outusing conventional analytical procedures. In order to determine thesurface areas and noble metal concentration at particular depths withinthe catalyst particle, a method called the chloroform attrition methodis employed. This method involves the agitating of a certain weight ofcatalyst in a liquid (chloroform) for a specified length of timedependent upon the amount of surface to be attrited off. The attritedmaterial is separated from the unattrited remainder, dried and weighed.Knowing the initial dimensions of the catalyst particles as well astheir geometry and weight, the depth removed can be determined. Thisattrited material is analyzed for its noble metal surface area and noblemetal content. From these data the noble metal surface area and noblemetal contents can be calculated as a function of depth into thecatalyst particle.

The activity and durability of the catalysts were ultimately tested byseveral methods. Bench scale activity testing is carried out by a methodwhich simulates the cold start that a catalyst experiences on a vehicle.Once the catalyst heats up, after a period of time it will reachessentially a steady state condition whereupon the efficiencies attain alevel dependent upon the intrinsic activity of the catalyst. In thebench scale test, a 13 cm.³ sample is contacted with sufficient totalgas flow rate to achieve a gas hourly space velocity of 38,000 hr.⁻¹.The simulated exhaust contains 1700 ppm carbon as propane, 4.5 volumepercent carbon monoxide with the balance made up by nitrogen. Thepreheated gas, if containing no oxidizable species, would heat up thebed to 700° F. However, due to the presence of carbon monoxide andhydrocarbon, the temperature climb in the bed is accelerated due to theexothermic oxidation reactions. In this test the parameters ofimportance are the rapidity of lightoff as measured by the time to reach50% conversions of carbon monoxide and hydrocarbon and the carbonmonoxide and hydrocarbon conversion efficiencies which occur when thecatalyst reaches essentially a steady state condition. The catalysts ofthis invention exhibit very short times to reach 50 percent conversionsand have very high conversions of hydrocarbon and carbon monoxide.

In order to assess the durability of catalysts upon exposure to fueladditives which contain lead, sulfur, and phosphorus along with thephenomena of varying temperature conditions, the catalysts are aged on apulse flame combustor system. In this system a fuel doped with all thepoison precursors one finds in current fuels is burned resulting in thedeposition of poisons such as lead, sulfur and phosphorus compounds onthe catalyst. The fuel which is hexane doped with tetramethyl leadantiknock mix, trimethyl phosphite and thiophene contains the equivalentof 0.23 g. lead per gallon, 0.02 g. phosphorus per gallon and 0.03weight percent sulfur. The fuel flow is 15 ml. per hour. Nitrogen andoxygen are mixed in amounts of 2000 and 500 standard cubic centimetersper minute, respectively. An additional 50 standard cubic centimeters ofoxygen is added after the point of combustion to ensure an oxidizingenvironment. A detailed explanation of the operation of a pulse flamecombustor operation was presented by K. Otto, R. A. Dalla Betta and H.C. Yao, J. Air Pollution Control Association, Vol. 24, No. 6, pp.596-600, June 1974. A 13 cm.³ sample is exposed to repetitivetemperature cycling consisting of one hour at 1300° F. and 2 hours at1000° F. Temperatures are the average bed temperatures. During prolongedaging periods of up to 500 hours the sample is periodically checked foractivity by removing it from the pulsator unit and testing it on thebench scale activity unit. The small scale activity and aging tests arevaluable in screening many catalysts; however, the ultimate test is thefull size vehicle or engine dynamometer evaluation. A detaileddescription of an engine dynamometer test is reported by D. M. Herod, M.V. Nelson and W. M. Wang, Society of Automotive Engineers, Paper No.730557, May 1973. This test is similar to the bench scale activity testreported earlier. The ambient temperature catalyst contained in a fullsize converter is contacted with the hot exhaust from a closelycontrolled engine. The conversions of carbon monoxide and hydrocarbonand nitrogen oxides are monitored as a function of time. Correlationshave been developed which allowed prediction of how the catalyst wouldperform if tested in an actual CVS test run by the Federal TestProcedure as detailed in the Federal Register of July, 1970 and asmodified by the instructions in the Federal Register of July 2, 1971.Durability testing of full size converter charges of catalysts iscarried out by the method outlined by J. P. Cassassa and D. G. Beyerlin,Society of Automotive Engineers, Paper No. 730558, May, 1973.

Once the catalyst has passed laboratory and engine dynamometer activityand durability testing, it then undergoes fleet testing on standardproduction type vehicles. The vehicles are tested according to theFederal Test Procedure noted previously and hereby incorporatedreference. The procedure is designed to determine the hydrocarbon,carbon monoxide and oxides of nitrogen in gas emissions from anautomobile while simulating the average trip in an urban area of 71/2miles from a cold start. The test consists of engine start up andvehicle operation on a chassis dynamometer through a specified drivingschedule consisting of a total of 1,371 seconds. A proportionate part ofthe diluted gas emissions is collected continuously for a subsequentanalysis using a constant volume sampler.

The dynamometer run consists of two tests, a cold start test after aminimum of 12 hours wait, and a hot start test with a 10-minute waitbetween the two tests. Engine start up and operation over a drivingschedule and engine shutdown constitute the complete cold start test.Engine start up and operation over the first 505 seconds of the drivingschedule complete the hot start test.

The engine emissions are diluted with air to a constant volume and aportion sampled in each test. Composite samples are collected in bagsand analyzed for hydrocarbons, carbon monoxide, carbon dioxide andoxides of nitrogen. Parallel samples of diluted air are similarlyanalyzed for hydrocarbons, carbon monoxides and oxides of nitrogen.Vehicle aging of the catalyst is carried out following the drivingschedule as set forth in Appendix "D" schedule in the Federal TestProcedures, noted previously. This schedule consists of eleven 3.7 milelaps of stop and go driving with lap speeds varying 30-70 mph., repeatedfor 50,000 miles. The average speed is 29 mph. Periodically the vehiclesare tested on a chassis dynamometer to assess the durability of theemission control system.

Catalysts designed for three-way control of carbon monoxide,hydrocarbons and nitrogen oxides require a support of good physicalintegrity because of the variation in reducing or oxidizing nature ofthe exhaust environment. Furthermore, they especially require a supporthaving low density and a high degree of macroporosity because of theinherent difficulty in achieving good carbon monoxide removal and thequick lightoff needed for good all-round performance. Since rhodium istypically the catalytic agent added for improved three-way control andcoupled with its being a rather scarce resource, it must be usedefficiently and placed strategically. Similar to the case of oxidationcatalysts, three-way catalysts have high initial activity as well asgood sustained performances when the active metals are properlypositioned. We have found that for combined high initial activity andsustained catalytic durability, the noble metals should be located suchthat about 50% of the total noble metal lies deeper than about 75microns.

Laboratory determination of three-way catalyst activity is made bycontacting 13 cm.³ of the catalyst with sufficient gas flow to reach agas hourly space velocity of 40,000 hr⁻¹. The gas composition consistsof 1% carbon monoxide, 250 ppm hydrocarbon consisting of a 3/1 mixtureof propylene and propane, 0.34% hydrogen, 1000 ppm NO, 12% carbondioxide, 13% water and varied oxygen contents to change the exhaust gasenvironment from reducing to oxidizing in nature. Nitrogen is added asthe balance. The measure (φ) of reducing or oxidizing nature of theexhaust gas environment is given by the following: ##EQU1##

The catalyst is evaluated at various values of φ and at various steadystate temperatures.

Bench scale durability is carried out in essentially the same way as inthe case of oxidation catalysts. Periodic activity checks are run duringthe aging schedule.

Full scale engine or chassis dynamometer evaluation in technically andeconomically feasible systems is carried out with close control ofair/fuel ratios. Aging is carried out in systems with similar control ofthe exhaust gas environment to which the catalyst is exposed. Periodicchecks of three-way performance are made during the aging schedule.

The catalysts of this invention have been tested by a number of methodswhich have been developed by the automobile manufacturers to measurecatalytic performance. One particular test measures conversionefficiency at a temperature of 1000° F. and a gas hourly space velocityof 75,000 hour⁻¹. The test is particularly discriminative in determiningthe relative ability of catalysts to oxidize hydrocarbons. Below aretypical ranges of performance for highly preferred catalysts of ourinvention compared to those achieved by typical catalysts of currentmanufacture.

    ______________________________________                                        Efficiency                                                                                             Aged 24 Hours                                                          Fresh  1800° F.                                      ______________________________________                                        Catalysts of this Invention                                                                       64- 65%  40- 44%                                          Catalysts of                                                                  Current Manufacture (Typical)                                                                     38- 41%  about 35%                                        ______________________________________                                         HC Efficiency = hydrocarbon conversion efficiency at steady state        

Another useful test that has been used to differentiate catalysts of ourinvention from those of current manufacture is one that measures thetemperatures at which fifty percent of the carbon monoxide in the testgas mixture is oxidized at a gas hourly space velocity of 1400 hour⁻¹.Low temperature results mean high activity. Below are typical ranges ofperformance of highly preferred catalysts of our invention compared tothose of typical catalysts of current manufacture:

    ______________________________________                                                    Temperature for 50% Carbon                                                    Monoxide Conversion                                                           Fresh   Aged 24 hrs. at 1800° F.                           ______________________________________                                        Catalysts of                                                                  our Invention 225°-230° F.                                                              265°-300° F.                            Catalysts of                                                                  Current Manufacture                                                                         300°-315° F.                                                              350°-370° F.                            ______________________________________                                    

Because of the low density and macroporosity of the spheroids of thisinvention, the catalytic agents, for example, the platinum group metals,when applied to specific locations and in specific distributions withinthe catalyst can be utilized very efficiently. Because of this efficientusage, the catalyst need not be loaded with noble metals to a levelwhich exceeds its economic limits. Hence, the ranges of total noblemetal loading are:

    ______________________________________                                                      Broad Range Normally                                            ______________________________________                                        Total Noble Metal                                                             Loading Weight Percent                                                                        0.005-1.0     0.03-0.30                                       ______________________________________                                    

The particular choice of catalytic agents used depends upon theperformance characteristics desired in the system. Principally, thenoble metals used in automobile exhaust emission control are platinum,palladium, and rhodium and mixtures thereof.

The approximate ranges of these principal components are:

    ______________________________________                                                                 Normally                                                           Broad Range                                                                              Preferred                                            ______________________________________                                        Platinum (% of total)                                                                         0-100%       65-75%                                           Palladium (% of total)                                                                        0-100%       25-35%                                           Rhodium (% of total)                                                                          0-20%         5-15%                                           ______________________________________                                    

Because of the high degree of macroporosity built into the support, thenoble metals may be positioned deeper than in typical catalysts ofcurrent manufacture, and as a result they are highly dispersed and moreresistant to crystallite growth. Hence, the catalysts are characterizedas having high and stable metal surface areas. The following rangesdistinguish advanced catalysts of our invention from typical catalystsof current manufacture:

    ______________________________________                                        Noble Metal Surface Area (micromoles of                                       H.sub.2 per gram of catalyst with a metal loading                             of 0.332 troy ounces/ft..sup.3)                                                                      Aged 24 Hours                                                     Fresh       1800° F.                                        ______________________________________                                        Catalysts of this                                                             Invention                                                                       Broad Typical                                                                            3.8-7.6       0.5-0.7                                            Catalysts of                                                                  Current                                                                       Manufacture                                                                     Broad Typical                                                                            0.6-3.5       0.0-0.1                                            ______________________________________                                    

The catalysts of our invention are further characterized by the specificdepths to which the catalytic agents are deposited. There are pronounceddifferences between the catalysts of our invention compared to typicalcatalysts of current manufacture as noted below:

    ______________________________________                                        Approximate Maximum                                                           Depth of Noble Metal Penetration                                              Catalysts of our Invention                                                      Broad Typical      150-400 microns                                            Preferred          150-250 microns                                          Catalysts of Current Manufacture                                                Broad Typical       30-125 microns                                          ______________________________________                                    

Because of the specific performance characteristics that we build intoour catalyst by changing the distribution and location of the variousnoble metals, those distributions and locations are distinguishingcharacteristics of the catalyst of our invention. Although the overallproportions of the catalyst as a whole may be fixed, the distribution ofthese components is specifically located in the catalysts of ourinvention. The following indicates the preferred ranges of depths anddistributions that characterize the catalysts of our invention:

    ______________________________________                                                    Approximate Maximum                                                           Depth of Penetration                                              ______________________________________                                        Platinum                                                                        Broad Typical                                                                             125-400 microns                                                   Preferred   125-250 microns                                                 Palladium                                                                       Broad Typical                                                                             125-400 microns                                                   Preferred   125-250 microns                                                 Rhodium                                                                         Broad Typical                                                                             125-250 microns                                                   Preferred   125-200 microns                                                 ______________________________________                                    

Catalysts of our invention are characterized by the followingperformance characteristics which distinguish them from typicalcatalysts of current manufacture.

    ______________________________________                                        Fresh Laboratory Dynamic                                                      Heat-up Activity                                                                           .sup.t 50 CO HC Efficiency                                       Parameter    Typical      Typical                                             ______________________________________                                        Catalysts of                                                                  our Invention                                                                              40-50 sec.   75-95%                                              Catalysts of                                                                  Current Manu-                                                                 facture      >60          55-75%                                              ______________________________________                                         .sup.t 50 CO = time in seconds for 50% carbon monoxide conversion.       

    Laboratory Dynamic Bench Heat-up                                              Activity after 500 hours of Pulsator Aging                                                 .sup.t 50 CO HC Efficiency                                       Parameter    Typical      Typical                                             ______________________________________                                        Catalysts of                                                                  our Invention                                                                              65-95 sec.   35-50%                                              Catalysts of                                                                  Current Manu-                                                                 facture      >135 sec.    10-20%                                              ______________________________________                                    

The following examples illustrate specific embodiments of the invention.

EXAMPLE 1

This example illustrates the preparation of an alumina composition ofthis invention.

Alumina trihydrate was completely dissolved in sodium hydroxide toprovide a sodium aluminate solution containing 20 percent Al₂ O₃ andhaving a Na₂ O/Al₂ O₃ mole ratio of 1.40. 495 grams of water were addedto a reaction vessel and then 631 milliliters of 50 percent sodiumhydroxide solution were added. This volume of sodium hydroxide solutioncorresponded to 966 grams at the specific gravity of the solution of1.53 g./cm³. The mixture was stirred gently and heated to 200° F. Atotal of 672 grams of alumina trihydrate was added gradually over aperiod of 30 minutes. During the addition of the alumina trihydrate, themixture was heated to a gentle boiling and stirred slowly. Gentleboiling and stirring were then continued for another 60 minutes or untilall the trihydrate was dissolved. Heating was stopped and the mixturecooled with stirring to 140° F.

The specific gravity and temperature of the sodium aluminate solutionwere adjusted to 1.428 g./cm.³ and 130° F. respectively by adding 290grams of water at a temperature of 140° F. and stirring the mixture.2016 grams of the solution were used for the preparation of the alumina.

2286 grams of an aluminum sulfate solution containing 7 percent Al₂ O₃and having a specific gravity of 1.27 g./cm.³ at 25° C. and a SO₄ ═/Al₂O₃ mole ratio of 3.01 were prepared by dissolving 1373 grams of aluminumsulfate crystals in 1963 grams of water.

The sodium aluminate solution and the aluminum sulfate solution wereheated to 145° F. A heel of 3160 grams of water was placed in a striketank, the agitator was started, and the heel heated to 155° F.

The heel was acidified to a pH of 3.5 by the addition of 6 millilitersof aluminum sulfate at an addition rate of 36 ml./minute and aged for 5minutes. At the conclusion of the aging period, the flow of sodiumaluminate was started at a rate of 28 ml./minute. Within 5 seconds, theflow of aluminum sulfate was resumed at 36 ml./minute and maintainedconstant through the 50 minute strike phase. The flow of sodiumaluminate was adjusted as needed to maintain the pH of the reactionmixture at 7.4. The strike temperature was maintained at 163° F. byheating the strike tank.

In 50 minutes, all of the aluminum sulfate solution had been added and317 grams of sodium aluminate remained.

At the conclusion of the strike, the pH of the reaction mixture wasincreased to 10.0 by adding 29 more grams of sodium aluminate solution.The final molar ratio of Na₂ O to SO₄.sup.═ was 1.00. The solution wasstabilized by aging for 30 minutes at a constant temperature of 163° F.

After aging, the reaction mixture was filtered and washed. For everygram of alumina in the mixture, 50 grams of wash water were used. Astandard filtration-wash test was defined as follows. Reaction slurry(600 ml.) was filtered in an 8 inch diameter crock using Retel filtercloth, material no. 80, at 10 inches of vacuum. It was washed with 2.5liters of water. The filtration time was 2.1 minutes and the filter cakewas 7 mm. thick.

The filter cake was reslurried at 15% solids and spray-dried at anoutlet temperature of 250° F. to a powder having a total volatiles(T.V.) content of 27.5%, as measured by loss on ignition at 1850° F. Thedried powder was calcined at 1850° F. for 1 hour.

The properties of the dry product and the calcined product are shown inTable 1.

                  Table 1                                                         ______________________________________                                        Dry Powder                                                                    ______________________________________                                        Wt. % Na.sub.2 O 0.02                                                         Wt. % SO.sub.4.sup.=                                                                           0.20                                                         Wt. % T.V.       27.5                                                         Agglomerate Size 21.5 μ                                                    Bulk Density     24.1 lbs./ft..sup.3                                          X-Ray Phases     (boehmite-pseudoboehmite                                                      intermediate)-no alpha or                                                     beta trihydrate phases                                                        present peak for [020] cry-                                                   stallographic plane falls at                                                  d spacing of 6.37 A.                                         Calcined Powder at 1850° F.                                              For 1 Hour                                                                  N.sub.2 Surface Area                                                                           136 m..sup.2 /g.                                             N.sub.2 Pore Volume, <600 A                                                                    0.72 cm..sup.3 /g.                                             total          0.95 cm..sup.3 /g.                                           X-Ray Phases     theta alumina, no alpha                                                       alumina present                                              Pore Size Distribution                                                                         A nitrogen PSD measurement                                                    showed that all the pores                                                     were greater than 100 A                                                       diameter and that 50% of                                                      the pores were in the 100-200 A                                               diameter range.                                              ______________________________________                                    

EXAMPLE 2

Given below in Table 2 is a summary of results of 13 runs using theprocess conditions described in Example 1.

                  Table 2                                                         ______________________________________                                        Properties         Average                                                    ______________________________________                                        No. of Runs        13                                                         Wt. Al.sub.2 O.sub.3 /Run (lbs.)                                                                 1                                                          Strike Ratio-Na.sub.2 O/SO.sub.4.sup.=                                                           0.93                                                       Standard Filtration Test (min.)                                                                  2.4                                                        Spray Dried Powder                                                            Wt. % Na.sub.2 O   0.03                                                       Wt. % SO.sub.4.sup.=                                                                             0.19                                                       Wt. % T.V.         27.9                                                       Bulk Density (lbs./ft..sup.3)                                                                    24.0                                                       N.sub.2 Surface Area at 750° F.                                        for 30 minutes (m..sup.2 /g.)                                                                    420                                                        N.sub.2 Pore Volume at 750° F.                                         for 30 minutes (cm..sup.3 /g.)                                                                   0.82                                                       <600 A                                                                        X-Ray              Intermediate Boehmite-                                                        Pseudoboehmite                                             Calcined Powder at 1850° F.                                              for 1 Hour                                                                  Surface Area (m..sup.2 /g.)                                                                      131                                                        Pore Volume (cm..sup.3 /g.)                                                     Total            1.01                                                         <600 A           0.73                                                       X-Ray              Theta alumina, no alpha                                                       alumina present                                            ______________________________________                                    

EXAMPLE 3

Given below in Table 3 is a summary showing the results for the blendedproducts of six large scale runs. The process was the same as in Example1 except that 195 lbs. of alumina (dry basis) were made per run.Equipment size and amounts of material were scaled up proportionately.The results were the same as in laboratory scale runs showing that theprocess could be readily scaled up.

                  Table 3                                                         ______________________________________                                        Spray Dried Powder                                                            ______________________________________                                        Wt. % Na.sub.2 O  0.059                                                       wt. % SO.sub.4.sup.=                                                                            0.31                                                        Wt. % CO.sub.2    1.37                                                        Wt. % T.V.        29.6                                                        Bulk Density (lbs./ft..sup.3)                                                                   30.0                                                        N.sub.2 Surface Area at 750° F.                                                          413.                                                        (m..sup.2 /g.)                                                                N.sub.2 Pore Volume at 750° F.                                                           0.77                                                        (cm..sup.3 /g.) <600 A                                                        X-Ray Phases      Intermediate Boehmite-                                      Calcined Powder-  Pseudoboehmite                                               1850° F./1 Hr.                                                        N.sub.2 Surface Area (m..sup.2 /g.)                                                             131                                                         N.sub.2 Pore Volume (cm..sup.3 /g.)                                            Total            0.97                                                         <600 A           0.70                                                        X-Ray Phases      Theta alumina, no alpha                                                       alumina present                                             ______________________________________                                    

EXAMPLE 4

The process conditions shown in Example 1 were important to obtain aneasily filterable, pure product. In runs where the process conditions ofExample 1 were employed except that the reaction temperature and timewere varied, the following results were obtained.

                  Table 4                                                         ______________________________________                                        Reaction Temperature                                                                          75° F.                                                                          120° F.                                                                        163° F.                                                                50 min.                                      Reaction Time   50 min.  25 min. (Example 1)                                  ______________________________________                                        Strike Ratio (Na.sub.2 O/SO.sub.4.sup.=)                                                       0.76    0.77    0.91                                         Standard Filtration Time                                                                      2.8      9.0     2.1                                          Test (min.) φ                                                             Wt. % SO.sub.4.sup.=                                                                          9.5      0.19    0.20                                         ______________________________________                                    

Thus, a decrease in process temperature led to an increase in sulfatecontent. A decrease in process time leads to an increase in filtrationtime.

EXAMPLE 5

This example illustrates the treatment of a washed alumina filter cakeprepared in accordance with the procedure of Example 3 with acetic acidbefore spray drying. The acetic acid has the effect of decreasing theabsorption of carbon dioxide during spray drying. In each run glacialacetic acid was added to the filter cake to a pH of 6.0 and the mixtureagitated. The spray dried product contained 3.8 percent acetic acid and64.5 percent alumina. This represents 0.1 moles of acetate ion for eachmole of alumina.

The properties of the alumina products of Example 3 and this Example areshown in Table 7. The carbon dioxide content is 0.76% compared to 1.37%present in the alumina of Example 3.

                  Table 5                                                         ______________________________________                                        Spray Dried Powder                                                                           Example 3    Example 5                                         ______________________________________                                        Wt. % Na.sub.2 O                                                                              0.059        0.053                                            Wt. % SO.sub.4.sup.=                                                                         0.31         0.50                                              Wt. % Solids (Al.sub.2 O.sub.3)                                                              63.8         64.5                                              Wt. % CO.sub.2 1.37         0.76                                              X-Ray          Intermediate Intermediate                                                     Boehmite-    Boehmite                                          Calcined Powder                                                                              Pseudoboehmite                                                                             Pseudoboehmite                                    1850° F./1 Hr.                                                         N.sub.2 Surface Area (m..sup.2 /g.)                                                          131          118                                               N.sub.2 Pore Volume (cm..sup.3 /g.)                                                          0.70         --                                                 <600 A                                                                       X-Ray          theta alumina,                                                                             theta alumina,                                                   no alpha     no alpha                                                         alumina      alumina                                                          present      present                                           ______________________________________                                    

EXAMPLE 6

In order to illustrate the relative proportions of crystalline materialand amorphous material in the alumina of this invention, samples of thealumina of Example 1 and aluminas A and B that exhibit lower and higherdegrees of crystallinity respectively were slurried with deionized waterat Al₂ O₃ concentrations of 100 g. Al₂ O₃ (dry basis) in one liter ofwater. Potentiometric titrations of each slurry were slowly conducted ata rate of addition of 1.1 N sulfuric acid of 1 ml./minute over the pHrange of 8.3 to 4.0 in which alumina is insoluble.

                  Table 6                                                         ______________________________________                                                   Volume of 1.1N H.sub.2 SO.sub.4 Solution Required                             To Reach Indicated pH                                                         Example 1 A         B                                              ______________________________________                                        d [020] spacing                                                                            6.37 A      6.56 A    6.11 A                                     midpoint width                                                                of peak [020]                                                                              1.78 A      1.98 A    0.18 A                                     pH                                                                            8.3           0           0         0                                         7.0          36           47       14                                         6.0          90          120       25                                         5.0          117         156       36                                         4.0          150         193       47                                         ______________________________________                                    

The results show that the alumina compositions of this inventionrequired intermediate amounts of acid to effect the same pH change andthus had a gel content intermediate between A and B.

EXAMPLE 7

The degree of crystallinity of the alumina compositions of thisinvention was further demonstrated by X-ray diffraction measurements ofthe development of beta alumina trihydrate on alkaline aging andheating. 100 gram samples (dry basis) of alumina A as shown in Table 6and the alumina prepared in Example 3 as shown in Table 3 were slurriedin 250 milliliters of deionized water and brought to pH 10 by theaddition of 1 N NaOH solution. The time and temperatures of aging andthe height of the high and low intensity X-ray peaks of alumina A forbeta trihydrate are shown in Table 7. No detectable beta trihydrate waspresent in the alumina composition of Example 3 under the sameconditions of aging and heating as alumina A.

                  Table 7                                                         ______________________________________                                                      4.72 A Peak   4.35 A Peak                                       Time / Temperature                                                                          Height (mm.)  Height (mm.)                                      ______________________________________                                        18 hrs. / 50° C.                                                                      8             8                                                24 hrs. / 50° C.                                                                     11             8                                                41 hrs. / 50° C.                                                                     10            12                                                 4 hrs. / 90° C.                                                                     12            14                                                21 hrs. / 90° C.                                                                     18            14                                                ______________________________________                                    

The ease of formation of beta trihydrate under alkaline conditions ofSample A indicated a higher gel content than in the alumina of Example3.

EXAMPLE 8

In a series of runs, five cubic feet of spheroidal alumina particleswith an average bulk density of 28 pounds per cubic foot were preparedusing a mixture of (1) a blend of the products of the runs of Example 3and (2) the acetic acid treated alumina of Example 5. An 80/20 mix ofplain alumina to acetate alumina was used and slurried in nitric acid,acetic acid, and water. The composition of the mixture was:

    ______________________________________                                        Example 3 Alumina                                                                           (63.8 Wt.% Al.sub.2 O.sub.3)                                                                    1919 g.                                       Example 5 Alumina 64.5 Wt.% Al.sub.2 O.sub.3                                                                        475 g.                                                    3.8 Wt.% H.sub.3 CCOOH                                                                             475 g.                                 1.5M HNO.sub.3                         600 ml.                                1.5M CH.sub.3 COOH                    1000 ml.                                Water                                 1780 ml.                                Nominal Composition of above mix.                                             (Al.sub.2 O.sub.3).sub.1.00 (CH.sub.3 COOH).sub.0.12 (HNO.sub.3).sub.0.06     (H.sub.2 O).sub.15.34                                                         ______________________________________                                    

The liquids were mixed together in a five gallon bucket and blended withthe alumina of Example 3 using a Cowles Dissolver with a three inchdiameter blade turning at 3500 R.P.M. A 20 minute blending was used. Theacetate alumina was then added and the slurry was blended for another 20minutes. Viscosity of the slurry immediately after blending was 78 cpsas measured with a Brookfield viscometer. The initial viscosity variedbetween 60 to 100 cps. The slurry was aged to a viscosity of 500 to 1000cps. before being used for sphere forming. After aging, the pH's of theslurry varied between 4.1 and 4.5 in the runs.

After aging, the alumina slurry was pumped to a pressurized feed tankthat was 5 inches in diameter and 4 inches high. The slurry wascontinuously circulated between the feed tank and a reservoir tank tomaintain the viscosity of the slurry. The alumina slurry feed flowedunder air pressure of 0.5 to 1.5 p.s.i.g. from the feed tank to thenozzle holder. The droplet formation rate varied between 140 and 170drops/minute. The nozzle holder could hold up to nineteen 2.7 mm.internal diameter nozzles in a regular array. 7 to 14 nozzles were usedper run and the extra openings in the nozzle holder were used as sparesin case any of the original nozzles clogged. The nozzle holder containedair channels to provide a linear air flow of 100 cm./minute around thenozzle tips and prevent ammonia vapor from prematurely gelling thealumina droplets.

The 2.7 mm. internal diameter of the droplet nozzle was selected to giveabout 1/8 inch diameter (minor axis) calcined spheres. The lip thicknesswas 0.6 mm. The 3.3 mm. nozzle holes opened to 1/2 inch diameter, 1/2inch long cylindrical holes cut in the bottom of the holder. The ends ofthe stainless steel nozzles were recessed 1/8 inch from the bottom ofthe holder and the bottom of the nozzle holder was 1/4 inch above theorganic phase of the column.

With a slurry viscosity of 700 cps. and a feed pressure of 1/2 p.s.i.g.a droplet rate of 170 drops per nozzle per minute could be maintainedusing seven nozzles. It took 11/4 hours to form the slurry batch intodroplets.

A glass sphere-forming column was employed. The column was 9 feet highand 4 inches in diameter. The column was filled with kerosene (no. 1grade) and 28% aqueous ammonia. The top six feet of the column containedthe kerosene. The remainder of the column was filled with the aqueousammonia. The aqueous ammonia was mixed with 1.3 wt.% ammonium acetateand 0.83 wt.% ammonium nitrate as measured under steady state operatingconditions. The kerosene was ammoniated to a concentration range of0.03-0.08 wt.% ammonia. The kerosene also contained 0.2 volume %Liquinox.

A glass column 4 feet high by 3 inches diameter was used to ammoniatethe kerosene. The column was half filled with ceramic saddles. Kerosenewas pumped from the top of the sphere forming column at a rate ofapproximately one liter per minute to the top of the ammoniating column.Ammonia gas flowed into the bottom of this column. The ceramic saddlesbroke up the stream of ammonia bubbles permitting a more efficientammoniation of the kerosene. Ammoniated kerosene was pumped from thebottom of the ammoniator column to the bottom of the kerosene layer inthe spheroid forming column. An ammonia concentration gradient existedwithin the kerosene phase of the spheroid forming column. The top of thekerosene phase had the least ammonia. The ammonia concentration at thetop of the kerosene phase was maintained between 0.03 and 0.08 wt.%. Theconcentration was determined by titration with HCl to a bromthymol blueendpoint.

A batch collection system was used. An 8 liter bottle was connected tothe bottom of the spheroid forming column by detachable clamps. A oneinch diameter ball valve was used to seal off the bottom of the columnwhen the collection bottle was detached. The collection bottle wasfilled with 28% aqueous ammonia. An overflow reservoir was connected tothe collection bottle to catch the aqueous ammonia displaced by thespheroids. When the collection bottle was full the spheroids were pouredinto a plastic basin where they were aged in contact with 28% aqueousammonia for one hour prior to drying.

A forced air drying oven was used. The spheroids were dried in nestingbaskets with a 20 mesh stainless steel screen bottom. The top of eachbasket was open to the bottom of the basket above it. The top basket wascovered. The bottom basket contained a charge of previously formedspheroids saturated with water. Because the sides of the baskets weresolid the flow of water vapor during drying was down and out through thebottom of the stack of baskets. A humid drying atmosphere was maintainedin this manner to prevent spheroid cracking. Drying temperature was 260°F. A 30 ft. long, gas fired tunnel kiln with a 14 inch square openingwas used to calcine the spheroids at 1900° F. for one hour.

A summary of the run conditions and properties of the calcined spheroidsfor this example are given in Table 8.

A summary of the properties of the approximately three cubic foot blendof calcined spheroids formed in a series of runs by the conditions ofthis example are shown in Table 9. The spheroid bulk density and averagecrush strength were relatively uniform. Attrition and shrinkage werelow.

                  Table 8                                                         ______________________________________                                        Slurry                                                                        ______________________________________                                        Wt. % Al.sub.2 O.sub.3 ; Nominal                                                                      26.5                                                          Actual*         27.6                                                  Blend Time (Min.)       20 + 20                                               pH                      4.39                                                  Aging Before Run (Hours)                                                                              4.5                                                   Run Viscosity; Initial (cps.)                                                                         700                                                   1900° F. Calcination                                                   Bulk Density (lbs./ft..sup.3)                                                                         28.2                                                  Crush Strength (lbs.)   9.0                                                     Range - High          11.5                                                  Low                     7.5                                                   Major Axis (mils)       148                                                   Minor Axis (mils)       130                                                   Major/Minor Axis ratio  1.14                                                  ______________________________________                                         *Water was evaporated during the mixing process.                         

                  Table 9                                                         ______________________________________                                        Weight (lbs.)   78.7                                                          Volume (ft..sup.3)                                                                            2.74                                                          Bulk Density (lbs./ft..sup.3)                                                                 28.7                                                          Average Crush Strength                                                                        10.5                                                          (lbs.)                                                                        % Attrition     0.5                                                           % Shrinkage     3.5                                                           Sphericity      1.13                                                          (Major Axis/Minor Axis)                                                       Average Diameter (mils)                                                                       135                                                           N.sub.2 Surface Area (m..sup.2 /g.)                                                           107                                                           X-Ray                                                                         Theta alumina, no alpha alumina-                                              ______________________________________                                    

EXAMPLE 9

This example illustrates the unique suitability of the aluminacompositions of this invention for the formation of spheroidal aluminaparticles.

In Table 10, the slurry and spheroid forming properties of threeconventional alumina powders are compared with those of three differentpowders produced by the powder formation process of Example 3. Theprocess described in Example 8 was used to produce the slurries andspheroids.

With both C and D alpha alumina monohydrate powders, it was necessary touse a lower solids content in the slurry. With a higher solids contentthan those used, the slurries set solid in a few minutes. Also, with theC alumina, it was necessary to use a higher alumina--acid ratio. At thestandard ratio, the slurry set up while blending.

The C and D aluminas resulted in high bulk density of spheroids.Although the same size nozzles were used in all cases, these twoaluminas formed spheroids which were much smaller than the spheroidsformed by the alumina compositions of this invention.

A crystalline alpha alumina monohydrate E was made by heating AlcoaC-30D alpha alumina trihydrate to 300° F. for 4 hours. A slurry made atthe standard alumina--acid ratio has a pH of 3.2. The solids settled outimmediately after blending. When the alumina--acid ratio was doubled (to1/0.09) the pH was 3.9, but the solids still settled out immediatelyafter blending.

                                      Table 10                                    __________________________________________________________________________    Powder              Example 3 Powders    C      D      E                      __________________________________________________________________________    Powder Properties                                                             % Total Volatiles   25.6   24.2   24.4   25.1   21.5   21.3                   Crystallographic Form                                                         - - - - - - - Alpha Alumina Monohydrate - - - - - - - - →              as is                                                                         Crystallographic Form,                                                        - - - - Theta Alumina - - - - - →               Kappa                  1900 ° F. for 1 Hour                            Alumina                N.sub.2 Surface Area, 1900° F.                                                             125    122    133    105    124    21                     for 1 Hour (m..sup.2 /g.)                                                     N.sub.2 Pore Volume, <600 A, (cm..sup.3 /g.)                                                      0.70   0.67   0.63   0.40   0.54   0.1                    at 1900° F. for 1 Hour                                                 Slurry Forming                                                                % Solids            27.6   27.5   32.0   19.1   23.4   30                     Mole Ratio; Al.sub.2 O.sub.3 /Acids                                                               1/0.18 1/0.18 1/0.18 1/0.09 1/0.18 1/0.18                 Slurry pH           4.4    4.5    4.1    4.5    4.5    3.2                    Initial Viscosity, (cps.)                                                                         84     320    400    400    220    5                      Time to reach 1000 cps.                                                                           4 hours                                                                              1 hour 2 hours                                                                              10 min.                                                                              1 hour Settled out            Sphere Properties After                                                       1900° F. calcination                                                   Crystallographic Form                                                         - - - - - - - - Theta Alumina - - - - - - - - →                        N.sub.2 Surface Area (m..sup.2 /g.)                                                               123    110    104    103    108                           N.sub.2 Pore Volume, <600 A (cm..sup.3 /g.)                                                       0.59   0.60   0.61   0.39   0.53                          Bulk Density        28.2   28.7   28.1   55.6   52.4                          Average Crush Strength (lbs.)                                                                     9.0    10.0   13.4   18.9   21.3                          Major Axis Diameter (mils)                                                                        148    141    155    102    116                           Minor Axis Diameter (mils)                                                                        130    118    121    92     98                            __________________________________________________________________________

The following examples illustrate the preparation and testing of thecatalyst of this invention.

EXAMPLE 10

An alumina slurry feed was made from an alumina powder which had thefollowing characteristics:

0.08 wt.% Na₂ O

0.43 wt.% SO₄ ═

0.095 wt.% CaO

0.022 wt.% MgO

29.4 wt.% Total Volatiles

0.85 cm.³ /g. N₂ Pore Volume

300 m.² /g. N₂ Surface Area at 1000° F.

X-ray diffraction shows alpha alumina monohydrate with the [020]reflection occurring at 6.6 A

The slurry had the following composition:

17.5 wt.% alumina

4.2 wt.% nitric acid (0.38 moles HNO₃ /mole Al₂ O₃)

The slurry was formed by hand stirring. It was aged for 2 days.Spheroids were formed in a 1-inch diameter column. Kerosene was thewater immiscible phase. The aqueous phase contained about 28 weightpercent ammonia. Three 175 g. batches were made and combined. Thesamples were calcined at 1000° F. for 3 hours. The properties of thespheroids were:

Bulk Density:--28.6 pcf.

Crush Strength:--13.2 lbs.

Water Pore Volume:--1.08 cm.³ /g.

Size:---6+7 mesh

After calcination at 1850° F. for 1 hour, it had:

Bulk Density:--34.3 pcf.

Crush Strength:--10.0 lbs.

Noble metal catalysts were prepared on the calcined substrates. Theconcentration was 0.04 troy oz./260 cu. in. in a 1 to 3 Pt/Pd weightratio. In a dynamic heat-up oxidation activity test, the followingresults were obtained:

    ______________________________________                                                    Fresh    24 Hrs./1800° F.                                  ______________________________________                                        CO Index      0.677      0.925                                                HC Efficiency, %                                                                            94.8       78.9                                                 ______________________________________                                    

These tests were conducted in accordance with the procedure of U.S. Pat.No. 3,850,847 to Graham et al, except that the simulated exhaust gascontained 1700 ppm carbon as propane.

The sample had excellent catalytic activity and stability for bothcarbon monoxide and hydrocarbon conversion.

EXAMPLE 11

Spheroidal alumina particles prepared in accordance with the procedureof Example 8 had the properties shown in Table 11.

1900 grams of these particles were impregnated to incipient wetness witha solution prepared as follows:

SO₂ was bubbled into 800 ml. deionized water for 17 minutes at 1 m.mole/minute after which 4.213 ml. of Pd (NO₃)₂ solution containing 105mg. palladium per ml. was added. The resulting solution is yellowishgreen indicating complexing of the palladium to a degree of 4 moles SO₂/g. atom palladium.

A solution of ammonium platinum sulfito salt, (NH₄)₆ Pt(SO₃)₄.xH₂ O, wasprepared by dissolving 3.678 g. having a platinum content of 30.67% in700 cc. water.

The palladium solution was then added to the platinum solution. Thetotal volume was then increased to 1938 ml. by the addition ofadditional deionized water. The solution was applied via a stream to therotating support. Once impregnation was complete, the support was placedon screens and oven dried at 320° F. (forced draft). After overnightdrying it was activated at 800° F. for 1 hour in air.

                  Table 11                                                        ______________________________________                                        Bulk Density       28.0                                                       Crush Strength (lbs.)                                                                            9.7                                                        Attrition %        0.20                                                       Sphericity                                                                    (Major Axis/Minor Axis)                                                                          1.1                                                        Surface Area (m..sup.2 /g.)                                                                      104.0                                                      X-Ray              theta alumina, no                                                             alpha alumina present                                      ______________________________________                                    

The bench and engine dynamometer, and vehicle test results for thiscatalyst are shown in Table 12 and the bench activity during aging onthe pulse flame combustor are shown in Table 13. The catalyst exhibitedhigh hydrocarbon conversion efficiency in the High Space Velocity test.It also was determined to have very low temperatures for 50% carbonmonoxide and hydrocarbon conversion in the static bench test. Thedynamometer aging data indicated good performance after 1000 hours ofengine aging. The catalyst was also aged on a vehicle in fleet tests andthe results were quite similar to those obtained in aging on a enginedynamometer. As observed in full scale engine tests, the light off(t50_(CO)) parameter as determined after extensive pulsator aging wasquite good.

In the tables which follow, the abbreviations used have the followingmeaning:

HSV=High Space Velocity

GHSV=Gas hourly space velocity

Cat.=Catalyst of the present invention

Std.=standard or reference catalyst typical of current commercialcatalyst in use in the U.S.A.

CVS=Constant volume sampling as per standard Fed. test procedure.

ND=Not determined

HC=Hydrocarbons

                  Table 12                                                        ______________________________________                                        Bench Test Results                                                            Fresh              Aged***                                                    HSV*        Static**   HSV        Static                                           HC      CO     HC    CO   HC    CO   HC    CO                            ______________________________________                                        Cat. 65      100    230   225  40    100  275   265                           Std. 38      100    311   313  35    100  369   370                           ______________________________________                                         *Conversion efficiency at 1000° F. 75,000                              **50% conversion temperature, 1400                                            ***Aged 24 hours at 1800° F.                                      

    Dynamometer Data - Fresh                                                      Time to 50% Conv.                                                                              600         Pred.                                            Seconds          Sec. Eff.   CVS Eff.                                              HC        CO        HC    CO    HC    CO                                 ______________________________________                                        Cat. 37        29        98    100   92    83                                 Std. 57        44        93     99   87    81                                 ______________________________________                                        Dynamometer Data - Aged 1000 Hours                                            Time to 50% Conv.                                                                              600         Pred.                                            Seconds          Sec. Eff.   CVS Eff.                                              HC        CO        HC    CO    HC    CO                                 ______________________________________                                        Cat.  97       72        79    97    78    77                                 Std. 115       87        76    95    74    74                                 ______________________________________                                        FIVE CAR FLEET TEST - DETAILS                                                 Vehicles:      5 cars - 350 V-8, 4 BBL. with                                                 M-Air                                                          MILEAGE                                                                       ACCUMMULATION: Rotate converters every 5000 miles,                                           (Every converter will be on each                                              car twice).                                                    Programmed                                                                    chassis dynamometer                                                           ______________________________________                                        1977 Schedule                                                                 FIVE CAR TEST FLEET                                                           10,000 MILE CATALYST EVALUATIONS                                              Predicted CVS Data                                                            HC Conversion Eff. CO Conversion Eff.                                              Veh. Aged  Dyno. Aged Veh. Aged                                                                              Dyno. Aged                                ______________________________________                                        Cat. 85         87         78       81                                        Std. 82         83         75       76                                        ______________________________________                                    

                  Table 13                                                        ______________________________________                                        BENCH ACTIVITY DURING                                                         AGING ON PULSE FLAME COMBUSTOR                                                                           Activity with                                      Total  Activity with Propane Feed                                                                        Propylene Feed                                     Aging  .sup.t 50 CO                                                                           .sup.t 50 CO                                                                           HC Eff. .sup.t 50 HC                                 Hours  (secs)   (secs)   (%)     (secs)                                       ______________________________________                                        0      54.3     95.4     79.9    54.0                                          69.5  66.0     ND       57.2    69.6                                         136.5  57.5     ND       51.1    76.8                                         203.5  76.8     --       38.8    96.9                                         ______________________________________                                    

Fuel: 0.23 g Pb/gal.; 0.02 g. P/gal.; 0.03% S

Catalyst underwent 152 cycles of 2 hours at 1000° F. and 1 hour at 1300°F.

Temperature is average axial bed temperature

EXAMPLE 12

Eleven 1300 gram batches of spheroidal alumina particles prepared inaccordance with the procedure of Example 8 and having the propertiesshown in Table 14 were impregnated as follows first with palladium,second with platinum.

                  Table 14                                                        ______________________________________                                        Average Bulk Density (lbs./ft..sup.3)                                                              28.8                                                     Crush Strength (lbs.)                                                                              8.7                                                       Average             12.2                                                      High                17.0                                                      Low                 9.0                                                      Attrition (%)        1.1                                                      Sphericity                                                                    (Major Axis/Minor Axis)                                                                            1.14                                                     Average Diameter (mils)                                                                            135                                                      Surface Area (m.sup.2 /g)                                                                          113                                                      X-Ray                theta alumina,                                                                no alpha alumina                                         ______________________________________                                    

The palladium solution was prepared by dissolving SO₂ at 2 m. moles/min.for 10 minutes in 800 ml. deionized water, after which 4.63 ml. Pd(NO₃)₂ at 105 mg./ml. palladium solution was added. To this 2.00 gramsof dibasic ammonium citrate were added, then the solution volume raisedto 1277 ml. It was impregnated to incipient wetness, then dried onscreens at 320° F. for a minimum of 11/2 hours, then dried at 500° F.overnight.

The platinum was then applied from a solution prepared by dissolving4.077 g. (NH₄)₆ Pt(SO₃)₄.xH₂ O at 30.67% Pt in 800 ml. of water and thenraising the impregnation volume to 1277 milliliters. The impregnatedsupport was dried on screens at 320° F., then activated at 800° F. for 1hour in air.

The bench activity during aging on the pulse flame combustor is shown inTable 15.

                  Table 15                                                        ______________________________________                                        BENCH ACTIVITY DURING                                                         AGING ON PULSE FLAME COMBUSTOR                                                                             Activity with                                    Total Activity with Propane Feed                                                                           Propylene Feed                                   Aging .sup.t 50 CO                                                                          .sup.t 50 HC                                                                          HC Eff. CO Eff.                                                                              .sup.t 50 HC                             Hours (secs)  (secs)  (%)     (%)    (secs)                                   ______________________________________                                        0     45.3     92.4   81.1    99.5   42.3                                     69.5  61.2    251.4   59.5    99.4   62.7                                     140.0 63.6    377.4   55.2    99.3   82.5                                     210.5 66.7    --      46.4    98.2   90.3                                     ______________________________________                                    

Fuel: 0.23 g Pb/gal.; 0.02 g. P/gal.; 0.03% S

Catalyst underwent 170 cycles of 2 hours at 1000° F. and 1 hour at 1300°F. Temperature is average axial bed temperature.

The bench and dynamometer test results for this catalyst are shown inTable 16.

                  Table 16                                                        ______________________________________                                        Bench Test Results                                                            ______________________________________                                        Fresh              Aged***                                                    HSV*        Static**   HSV        Static                                           HC      CO     HC    CO   HC    CO   HC    CO                            ______________________________________                                        Cat. 64      100    230   225  44    100  305   300                           Std. 41      100    302   303  35    100  351   351                           ______________________________________                                         *Conversion efficiency at 1000° F. 75,000 GHSV.sup.-1?                 **50% conversion temperature, 1400 GHSV.sup.-1?                               ***Aged 24 hours at 1800° F.                                      

    Dynamometer Data - Fresh                                                      ______________________________________                                        Time to 50% Conv.                                                                              600         Pred.                                            Seconds          Sec. Eff.   CVS Eff.                                              HC        CO        HC    CO    HC    CO                                 ______________________________________                                        Cat. 22        17        94    100   91    83                                 Std. 37        30        93    99    89    83                                 ______________________________________                                        Dynamometer Data - Aged 1000 Hours                                            ______________________________________                                        Time to 50% Conv.                                                                              600         Pred.                                            Seconds          Sec. Eff.   CVS Eff.                                              HC        CO        HC    CO    HC    CO                                 ______________________________________                                        Cat.  75       59        79    97    79    78                                 Std. 115       78        73    91    74    73                                 ______________________________________                                    

All of the test results (bench and engine) show the catalyst to beexcellent in fresh performance and very good in its ability to retainits activity.

EXAMPLE 13

The noble metal penetration in the catalyst of Example 12 was determinedby the chloroform attrition method and the results are shown in Table17.

These results indicate very deep penetration of both the platinum andthe palladium. The platinum was higher at the surface to ensure goodhydrocarbon performance, whereas the palladium was distributed veryuniformly to ensure good light off retention.

                  Table 17                                                        ______________________________________                                        Cumulative                                                                              Cumulative   Cumulative Cumulative                                  Noble Metal                                                                             Depth Attrited                                                                             Platinum   Palladium                                   S.A. %    (microns)    (%)        (%)                                         ______________________________________                                        29         18          14          4                                          44         40          28         11                                          54         60          39         18                                          64         81          51         25                                          68        101          59         32                                          73        120          65         37                                          77        146          72         45                                          79        173          77         51                                          ______________________________________                                    

EXAMPLE 14

A catalyst was prepared by impregnating spheroidal alumina particlesthat were prepared in accordance with the procedure of Example 8 andthat had the properties shown in Table 18.

                  Table 18                                                        ______________________________________                                        Bulk Density (lbs./ft..sup.3)                                                                   27.1                                                        Crush Strength (lbs.)                                                                           11.4                                                        Sphericity                                                                    (Major Axis/Minor Axis)                                                                         1.32                                                        Surface Area (m..sup.2 /g.)                                                                     112                                                         X-Ray             theta alumina, no                                                             alpha alumina present                                       ______________________________________                                    

100 cc. (43.7 g.) of the particles were impregnated to incipient wetnesswith a solution prepared by dissolving 59 mg. of (NH₄)₆ Pd(SO₃)₄. xH₂ O(containing 17.84% palladium) and 90 mg. of (NH₄)₆ Pt(SO₃)₄.xH₂ O(containing 29.28% platinum) in 42 ml. water. After impregnation, thecatalyst was dried on a screen at 320° F. in a forced draft oven. It wasthen activated at 800° F. in air for one hour. This catalyst exhibitedoutstanding fresh and pulsator aged performance. In particular, it hadgood light off, for example, as shown by very stable t₅₀ CO values.

                  Table 19                                                        ______________________________________                                        Bench Activity During                                                         Aging on Pulse Flame Combustor                                                                           Activity with                                      Total  Activity with Propane Feed                                                                        Propylene Feed                                     Aging  .sup.t 50 CO                                                                           t.sub.50 HC                                                                            HC Eff. .sup.t 50 HC                                 Hours  (secs)   (secs)   (%)     (secs)                                       ______________________________________                                        0      46.2      66.0    89.5    45.9                                         70     58.5     160.8    69.0    57.9                                         137.5  62.1     185.7    64.7    74.4                                         206.5  57.6     337.5    56.1    63.6                                         ______________________________________                                    

Fuel: 0.23 g. Pb/gal.; 0.02 g. P/gal.; 0.03% S

Catalyst underwent 160 cycles of 2 hours at 1000° F. and 1 hour at 1300°F.

Temperature is average axial bed temperature.

EXAMPLE 15

A catalyst was prepared by impregnating spheroidal alumina particlesthat were prepared in accordance with the procedure of Example 8 andthat had the properties shown in Table 20.

                  Table 20                                                        ______________________________________                                        Bulk Density (lbs./ft..sup.3)                                                                   29.7                                                        Crush Strength (lbs.)                                                                           9.2                                                         Sphericity                                                                    (Major Axis/Minor Axis)                                                                         1.16                                                        Attrition loss (%)                                                                              0.25%                                                       Surface Area (m..sup.2 /g.)                                                                     105                                                         X-Ray             theta alumina, no                                                             alpha alumina present                                       ______________________________________                                    

100 cc. (49.02 grams) of the particles were impregnated to incipientwetness with a solution prepared by bubbling SO₂ at 1 m. mole/min. for20 seconds into 10 milliliters of water, to which was added 0.100 ml. ofPd(NO₃)₂ at 105 mg. Pd per ml. To this solution was then added 0.682 ml.of acid platinum sulfito complex prepared by cation exchanging of (NH₄)₆Pt (SO₃)₄.×H₂ O which contained 38.6 mg. Pt/ml. Total impregnationvolume was increased to 42 ml. After impregnation the sample was placedon a screen and forced draft oven dried at 320° F. The sample wasactivated at 800° F. in air for 1 hour.

The catalyst contains 0.05 oz./ft.³ total noble metals at 5/2 Pt/Pdratio. The penetration depth was 25 to 50 microns.

The bench activity data for this catalyst are shown in Table 21. Thecatalyst performance is good but not nearly as is observed on catalystswith deeper penetrations.

                  Table 21                                                        ______________________________________                                        BENCH ACTIVITY DURING                                                         AGING ON PULSE FLAME COMBUSTOR                                                Total  Activity with Propane Feed                                             Aging  .sup.t 50 CO                                                                            .sup.t 50 HC                                                                            HC Eff.  CO Eff.                                   Hours  (secs)    (secs)    (%)      (%)                                       ______________________________________                                         0     52.0      72.9      90.6     99.5                                       70    90.6      --        30.9     99.3                                      146    99.5      --        25.2     98.9                                      ______________________________________                                        Total     Activity with Propylene Feed                                        Aging     .sup.t 50 HC                                                                             HC Eff.     CO Eff.                                      Hours     (sec)      (%)         (%)                                          ______________________________________                                         0         52.5      99.5        99.7                                          70       121.2      97.2        99.2                                         146       116.1      97.2        99.2                                         ______________________________________                                    

Test terminated due to rapid loss in propane efficiency

Fuel: 0.23 g. Pb/gal.; 0.02 g. P/gal.; 0.03% S

Catalyst underwent 49 cycles of 2 hours at 1000° F. and 1 hour at 1300°F.

Temperature is average axial bed temperature

EXAMPLE 16

A three way catalyst was prepared on the support described in Table 22

                  Table 22                                                        ______________________________________                                        Average Bulk Density (lbs./ft..sup.3)                                                              29.8                                                     Crush Strength (lbs.)                                                                              9.3                                                      Attrition %          0.14                                                     Sphericity           1.23                                                     (Major Axis/Minor Axis)                                                       X-Ray                theta alumina,                                                                no alpha alumina                                                              present                                                  ______________________________________                                    

Two batches of 1300 g. (=2.724 liters) of support were impregnated asfollows:

The support was sprayed to 1/2 of incipient wetness using an atomizingnozzle with a solution prepared by bubbling SO₂ into 400 milliliters ofwater for 6.12 minutes at 2 m moles SO₂ /min. To this 2.757 millilitersof Pd (NO₃)₂ solution at 105 mg. Pd/ml. were added. Then 1.284 g.ammonium citrate (dibasic) was added and volume increased to 610milliliters total. Immediately after palladium application, a solutionprepared by dissolving 2.2005 g. (NH₄)₆ Pt(SO₃)₄ ×H₂ O@ 32.88% platinumin 400 milliliters of water and then diluting to 610 milliliters wassprayed to the remainder of incipient wetness. It was dried at 320° F.for 2 hours and then at 500° F. for 1 hour. It was then sprayed to 95%of incipient wetness with a solution prepared by diluting acid Rhodiumsulfite solution which was prepared by cation exchanging (NH₄)₆ Rh(SO₃)₄×H₂ O using a cation exchange resin. 4.24 milliliters of acid Rhodiumsulfite @ 50.65 mg. Rhodium per ml. were diluted to 1160 milliliters andthen sprayed on the support. The impregnated catalyst was dried at 320°F. and was then activated at 600° F. for 1 hour.

Resultant total noble metal loading was 0.332 oz. total noble metal percubic foot of catalyst.

In Table 23, the results of three way catalyst testing are described.Considering the very small amount of rhodium present, the conversion ofnitrogen oxides to nitrogen was quite high which is attributed to theproper positioning of the rhodium in the spheroidal particles.

                  Table 23                                                        ______________________________________                                        NO.sub.x Conversion Efficiencies at Approx. 40,000 GHSV                                    φ - 0.70                                                                             φ = 0.95                                                         750°                                                                          900°                                                                          1050°                                                                       750°                                                                        900°                                                                        1050°                      Bed Temperature                                                                              F.     F.     F.   F.   F.   F.                                ______________________________________                                         Fresh                                                                         NO.sub.x →N.sub.2                                                                    64.8   75.0   87.5 97.7 97.7 97.7                               NO.sub.x (total)                                                                            98.9   98.9   100  98.9 98.9 98.9                              ______________________________________                                         FEED: 1% CO, 250 ppm. HC(C.sub.3 H.sub.6 /C.sub.3 H.sub.8 = 3/1), 0.34%       H.sub.2, 1,000 ppm. NO, 12% CO.sub.2, 13% H.sub.2 O, varying                  concentrations of O.sub.2, balance N.sub.2.                                   φ is a measure of air/fuel ratio defined as φ                         ##STR1##                                                                 

EXAMPLE 17

A catalyst was prepared on the spheroidal alumina particles of Table 18by impregnating platinum and palladium on different particles.

Two components were prepared; platinum on alumina, and palladium onalumina. They were prepared to provide a blend of 42.31% by weight Pdcomponent and 57.69% by weight Pt component that gave an equal atomloading on each support. The total noble metal loading is 0.332 oz. percubic foot.

The palladium particles were prepared by incipient wetness impregnationof 100 cc. (43.7 g.) of the support with a solution of (NH₄)₆ Pd(SO₃)₄×H₂ O@ 17.84% palladium dissolved in sufficient water to give a totalvolume of 42 milliliters. The impregnated support was dried at 320° F.and activated for 1 hour at 800° F.

The platinum particles were prepared by incipient wetness impregnationof 100 cc. (43.7 g.) of the support with a solution of (NH₄)₆ Pt(SO₃)₄×H₂ O@ 29.28% platinum dissolved in sufficient water to give a totalvolume of 42 milliliters. The impregnated support was dried at 320° F.and activated for 1 hour at 800° F.

The fresh activity and activity after 24 hours at 1800° F. is shown inTable 24 and the pulsator aging in Table 25. The fresh and thermal agedactivities were excellent. Pulsator aged performance was quite good.

                  Table 24                                                        ______________________________________                                                           Fresh                                                      t.sub.50.sbsb. CO (seconds)                                                                     44                                                          HC efficiency (%) 90                                                                            24 hours at 1800° F.                                 t.sub.50.sbsb.CO (seconds)                                                                      61                                                          HC Efficiency (%) 70                                                          ______________________________________                                    

                  Table 25                                                        ______________________________________                                        BENCH ACTIVITY DURING                                                         AGING ON PULSE FLAME COMBUSTOR                                                                    Activity                                                  Activity with Propane Feed                                                                          with Propylene Feed                                     Total                 HC    Co          HC    Co                              Aging t.sub.50 .sbsb.CO                                                                     t.sub.50 .sbsb.HC                                                                     Eff.  Eff.  t.sub.50 .sbsb.HC                                                                   Eff.  Eff.                            Hours (secs)  (secs)  (%)   (%)   (secs)                                                                              (%)   (%)                             ______________________________________                                         0    44.4     63.0   90.3  99.2  38.1  99.2  99.2                            69    56.2    138.0   72.6  99.3  58.5  98.9  99.2                            138.5 60.0    184.8   61.4  99.3  57.0  98.7  99.4                            209.0 63.7    531.3   52.6  99.3  69.9  98.0  99.3                            ______________________________________                                         Fuel: 0.23 g. Pb/gal.; 0.02 g. P/gal.; 0.03% S                                Catalyst Underwent 70 cycles for 2 hours at 1000° F. and 1 hour at     1300° F.                                                               Temperature is average axial bed temperature                             

EXAMPLE 18

The spheroids discussed in Example 8 were measured for nitrogen poresize and surface area distributions. The technique used is described byE. V. Ballou and O. K. Doolen in their article, Automatic Apparatus forDetermination of Nitrogen Adsorption and Desorption Isotherms, publishedin Analytical Chemistry, Volume 32, pp. 532-536 (April, 1960). Theequipment used for this determination was an Aminco Adsorptomatmanufactured by American Instrument Company of Silver Spring, Maryland.

The nitrogen BET surface area of this material was 120 m² /g with thefollowing distribution:

    ______________________________________                                                      Approximate % of Cumulative                                     Pore Diameter Nitrogen Surface Area to                                        (A)           Indicated Diameter                                              ______________________________________                                        600           1.3%                                                            500           1.6%                                                            400           2.3%                                                            300           5.0%                                                            200           16.3%                                                           150           46.4%                                                           100           about 100%                                                      ______________________________________                                    

It is obvious from these data that the vast majority of the pores werein the intermediate range of 100-1000 A. More specifically, over 80% ofthe pores were between 100 and 200 A, and that no surface area wasdetected by this technique below pores of 100 A in diameter.

What is claimed is:
 1. A process for preparing spheroidal aluminaparticles comprising:(a) commingling a precipitated alumina and anacidic aqueous medium to provide a slurry; (b) forming droplets of theslurry; (c) passing the droplets downwardly through air into an upperbody of water-immiscible liquid and ammonia and into a lower bodycomprising aqueous ammonia to form spheroidal particles; (d) aging theparticles in aqueous ammonia; and (e) drying and calcining the agedparticles.
 2. The process of claim 1 wherein the alumina comprises asubstantially pure, microcrystalline boehmite--pseudoboehmiteintermediate having from about 70 to about 85% weight percent of thetotal amount of Al₂ O₃ present in crystalline form.
 3. The process ofclaim 1 wherein the alumina has a [020] d spacing of about 6.2 to about6.5 A, and has a mid-point width of the [020] X-ray diffraction peak ofabout 1.65 to about 1.85 A.
 4. The process of claim 3 in which theslurry has a pH of about 4.0 to about 4.8 and a viscosity of about 200to about 1600 cps.
 5. The process of claim 3 in which the acidic aqueousmedium is selected from the group consisting of aqueous solutions ofnitric acid, hydrochloric acid, acetic acid, formic acid, aluminumnitrate and their mixtures.
 6. The process of claim 5 in which theslurry contains from about 10 to about 40 weight percent alumina andabout 1 to about 12 weight percent acid.
 7. The process of claim 6wherein the slurry has an acid to alumina molar ratio of about 0.05 toabout 0.50.
 8. The process of claim 3 in which the alumina and acidicaqueous medium are commingled by adding portions of the alumina to theacidic aqueous medium to acidify the alumina.
 9. The process of claim 1wherein the alumina comprises a microcrystalline boehmite-pseudoboehmiteintermediate having a nitrogen pore volume of about 0.60 to about 0.75cm.³ /g. and a surface area of about 100 to about 150 m.² /g., saidsurface area and pore volume being determined after a thermal treatmentfor about 1 hour at about 1850° F.
 10. The process of claim 1 whereinthe alumina comprises a microcrystalline boehmite-pseudoboehmiteintermediate in which about 130 to about 180 milliequivalents ofsulfuric acid per mole of alumina will change the pH of a water slurryof the alumina from about 8.3 to about 4.0.
 11. The process of claim 10wherein the alumina comprises a microcrystalline boehmite-pseudoboehmiteintermediate in which about 140 to about 160 milliequivalents ofsulfuric acid per mole of alumina will change the pH of a water slurryof the alumina from about 8.3 to about 4.0.
 12. The process of claim 1wherein the alumina comprises a substantially pure, microcrystallineboehmite-pseudoboehmite intermediate having a [020] d spacing of about6.3 to about 6.4 A., a nitrogen pore volume of about 0.64 to about 0.72cm.³ /g., and a surface area of about 110 to about 140 m.² /g., saidpore volume and surface area being determined after a thermal treatmentfor about 1 hour at about 1850° F.
 13. The process of claim 1 whereinthe alumina comprises a washed alumina filter cake.
 14. The process ofclaim 1 wherein the aged particles are calcined at about 1750° to about1950° F. for a period from about 30 minutes to about 3 hours.
 15. Theprocess of claim 14 wherein the aged particles are calcined at fromabout 1800° to about 1900° F. for a period from about 1 hour to about 2hours.
 16. The process of claim 1 wherein the alumina is prepared by aprecipitation in which aqueous solutions of sodium aluminate andaluminum sulfate are reacted so that the ratio of sodium expressed asmoles of Na₂ O to sulfate ion expressed as moles of equivalent H₂ SO₄ isin excess of 0.80 but below 0.97.
 17. The process of claim 16 whereinthe ratio is in excess of 0.88 but below 0.97.
 18. The process of claim1 wherein the alumina also comprises calcined alumina fines produced inthe process in an amount of up to about 15 percent of the total alumina.19. The process of claim 13 wherein the alumina also comprises calcinedalumina fines produced in the process in an amount of up to about 15percent of the total alumina.
 20. The process of claim 19 wherein theacidic aqueous medium comprises an aqueous solution of aluminum nitrate.21. The process of claim 1 wherein the acidic aqueous medium comprisesan aqueous solution of an acid salt, a monobasic acid, or a mixture ofmonobasic acids.
 22. The process of claim 1 wherein the acidic aqueousmedium comprises an aqueous solution of nitric acid and a decomposablemonobasic organic acid.
 23. The process of claim 22 wherein the organicacid is acetic acid, formic acid, or mixtures thereof.
 24. The processof claim 1 wherein the slurry contains about 24 to about 32 weightpercent alumina and has a viscosity of about 800 to about 1200 cps. 25.The process of claim 1 wherein the ammonia concentration in thewater-immiscible liquid is between about 0.01 to about 1.0 weightpercent.
 26. The process of claim 1 wherein the ammonia concentration inthe water-immiscible liquid is between about 0.04 to about 0.07 weightpercent.
 27. The process of claim 1 wherein water-immiscible liquid andammonia are introduced into the lower portion of the upper body, flowupwardly through the body in countercurrent flow with the droplets, andare removed at the top of the upper body.